I've come across this term recently and have seen quite a few opinions and definitions. My understanding is that it is actually a hardcore Theravada movement that is in some ways a conservative backlash against watered down Buddhism in the West. Is this a correct assessment?
The self proclaimed Arahant, Daniel Ingram & his book "Mastering the Core Teachings of the Buddha" is probably where the "hardcore" thing came from because the book covers supposedly "hardcore" and hard to find teachings like the "stages of insight" that are rarely covered in books. Dan seems very angry & ridiculous in certain rants in the book, I guess that is supposed to be some kind of "hardcore" teaching. I don't understand these rantings in an otherwise useful and informative book. Here's the link to his book that many have found useful:
Assuming Uilium's answer is correct you can get a bit of a sense of what "hardcore" might mean by reading the book's Forward and Warning.
He says he deliberately wrote it in his own voice, that some of what he writes (whole chapters of social commentary) "has a definite bite to it", and,
"it is the unrestrained voice of one from a generation whose radicals wore spikes and combat boots rather than beads and sandals, listened to the Sex Pistols rather than the Moody Blues, wouldn't know a beat poet or an early '60s dharma bum from a hole in the ground, ..."
"it is also the unrestrained voice of one whose practice has been dedicated to complete and unexcelled mastery of traditional and hardcore stages of the path rather than some sort of vapid New Age fluff or pop psychological head trip."
Hardcore is a (slightly dated) musical term, associated with punk, for example,
An article in Drowned in Sound argues that 1980s-era "hardcore is the true spirit of punk", because "after all the poseurs and fashionistas fucked off to the next trend of skinny pink ties with New Romantic haircuts, singing wimpy lyrics", the punk scene consisted only of people "completely dedicated to the DIY ethics". One definition of the genre is "a form of exceptionally harsh punk rock."
One of the important philosophies in the hardcore scene is authenticity. The pejorative term "poseur" is applied to those who associate with punk and adopt its stylistic attributes but are deemed not to share or understand the underlying values and philosophy. Joe Keithley, the singer for D.O.A. said in an interview that: "For every person sporting an anarchy symbol without understanding it there’s an older punk who thinks they’re a poseur."
I think that it would be more accurate to say not a "conservative backlash" but rather a "progressive backlash" against watered down pop-buddhism.
The simple definition of "hardcore dharma" to my knowledge is that it is dharma that focuses on a sincere and effortful aspiration to develop the skills of generosity, compassion, ethical training, meditative absorption and transformative lasting insight into No-self, in the style originally advocated by the Buddha, but also incorporating whatever modern skilful means necessary, be they from science, psychology, etc. Hence why its not a conservative backlash (but incorporates many of the partial valid truths found in conservative forms of buddhism, when applicable). I think most of the people who oppose the term hardcore dharma would be the conservative buddhists.
So the term refers refers to a particularly niche but world-wide philosophy of spiritual practice that is emerging, and is particularly linked to the world of neuroscience and the various dharma practitioners who are involved in that field, who are using their scientific training to inform their meditation practice, as well as the emerging-interfaith dialogue about how various practices across all the wisdom traditions share commonalities in their attitudes and techniques on how to transform human consciousness.
Also worth pointing out that Daniel's claim to Arahatship isn't entirely "self-proclaimed" and he writes about some of his Path attainments being confirmed by some high-Asian teachers (whose names I forget). To be a self-proclaimed Arahat also isn't necessarily entirely a bad thing (The Buddha himself was self-proclaimed as have many other figures, to be self proclaimed is actually to be expected, I think few people become enlightened without realising it...)
Having read his book, and with honesty having also reaped great benefits from it, I mainly associate hardcore Dharma with the words of Daniel M. Ingram (here a link to his personal website).
His way of teaching may initially seem a bit blunt, but that's just it and if I can recall correctly, Daniel makes it clear he is aware of it and explains the reasons for it in MTCB. As I have probably read or heard somewhere (see for example the playlist Pragmatic Dharma with Daniel Ingram at Buddhist Geeks) his is just an approach at dealing differently, and probably in a more modern way, with buddhist morality.
I'm not trying at all to advocate or even criticize the approach of practitioners and authors such as Daniel Ingram, but rather than interpreting hardcore Dharma teachings as conceited ways of spreading Buddhism, I would define them as teaching methods heavily focused on the "technical" parts, and always eager to spot and correct old Buddhism dogmas. To my own seeing, Daniel's style of following the teachings is more concerned with being critical and skeptical about what is taught than with establishing a mindless tranquility.
Beside Daniel Ingram's book Mastering the Core Teachings of the Buddha, freely available on his website in different formats, you can read more about what he did also on Dharma Overground (and perhaps even elsewhere, if you have any way of searching on the Web).
Householder Devindra, interested,
Ordaining and practicing, keeping the rules very strict, and tending toward the dhutaṅgas is not only in the west (outside of Theravada) but also in SEAsia seen as "hardcore"-practice.
Most Buddhist everywhere may see such as serious practicing 8 precepts and normal devotional practice, chanting and also giving food, alms and other to monks as hardcore.
Generally would the observing of the proper duties by "buddhists", Dhammika, for both, monks and lay-follower be considered as "hardcore" by more common faith follower.
Generally would accepting as certain hardships considered giving and enduring for the sake of purification as such considered.
Yet all of that is pretty normal in serious communities to the main parts of it.
Is this a correct assessment?, was asked at least:
Well, that depends on whether one perceives such as "insider" or "outsider", or even as one complete. As for the Buddha, there is nothing but praise for serious practice of which others could perceive.
May all tend soon, if not yet, to "hardcore" in relation to generosity and virtue so that the path would possible develope soon for paths and fruits, highest gain in release from all yokes.
(Note that this is not given for trade, exchange, stacks, entertainment and akusala deeds, but as a share of merits and continue such for release)
Hardcore Dharma is an branch of American Buddhism, like the self-proclaimed hardcorers in this video starting at 5:52. Instead of wearing saffron robes, hardcorers dress in black leather.