That's one of several translations of this
Yanta santaṃ padaṃ abhisamecca;
Sakko ujū ca suhujū ca,
Sūvaco cassa mudu anatimānī.
They all translate it with the same word, "upright".
The Pali word are ujū (straight), sakko (able/capable), and su- (good).
Here is a dictionary definition: https://suttacentral.net/define/uju
The dictionary has cross-references to other suttas where it's used. Curiously the first of these is
DN.iii.150 i.e. DN 30 where it seems to be used literally/physically -- e.g. a straight, well-formed body -- but there it's also used to describe eyes, behaviour, demeanour:
Mendicants, in some past lives the Realized One was reborn as a human being.
When looking at others he didn’t glare, look askance, or avert his eyes. Being straightforward, he reached out to others with straightforward intentions, looking at people with kindly eyes.