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In karaniya metta sutta,
He who is skilled in (working out his own) well being, and who wishes to attain that state of Calm (Nibbana) should act thus: he should be dexterous, upright, exceedingly upright, obedient, gentle, and humble.
What does it mean to be upright?
That's one of several translations of this
Karaṇīyamatthakusalena,
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.
What does it mean to be upright?
It means to go to great lenghts to follow Buddhist ethics as taught by the Buddha - at least keeping the 5 precepts if being a layperson.
araṇīyam'atthakusalena Yaŋ taŋ santaŋ padaŋ abhisamecca: Sakko ujū ca sūjū ca, Suvaco cassa mudu anatimānī.
Practicing "respectfully, carefully, duly, thoroughly", very, very... more in Karaṇīya Metta-Sutta
(Upright is something only reached by stream enter, while metta to practice is open for worldlings as well.)
