How can you translate or define nibbida in English?
The Pali Text Society Dictionary defines nibbidā as "weariness, disgust with worldly life, tedium, aversion, indifference, disenchantment."
Nibbidā occurs when one achieves yathābhūtañānadassana or "knowledge and vision of things as they are". By "things" we mean dhammas as the objects of mental activity. One sees sense experience as it is and becomes fed up with it (nibbidā). This in turn leads to virāga.
PTSD defines virāga as "absence of rāga, dispassionateness, indifference towards, disgust, absence of desire, destruction of passions; waning, fading away cleansing, purifying; emancipation, Arahantship."
Normally, we are intoxicated with sense experience. Once we have knowledge and vision, we sober up. Being fed up with sense experience, one is disenchanted and loses interest in it. And this leads to liberation (vimutti).
This sequence occurs very often in the Pāli suttas. It is also associated with the so-called Spiral Path which shows how paṭiccasamuppāda works to free us from suffering (See for example Aṅguttara Nikāya, Chp 10, suttas 1-5). For a survey of the Spiral Path texts in Pāḷi see The Spiral Path or Lokuttara Paṭiccasamuppāda, Western Buddhist Review. For the Chinese counterparts in the Madhymaāgama see Chinese Spiral Path Texts from the Madhyamāgama. The latter are also now available in professional translations: The Madhyama Āgama (Middle Length Discourses: Taishō 2 no.26) Editor in Chief Marcus Bingenheimer. Bukkyo Dendo Kyokai America Inc. 2013.