Lupus est homo homini, non homo, quom qualis sit non novit.
Man is no man, but a wolf, to a stranger.
Asinaria, Act II, scene 4 (line 495 of full Latin text).
Variant translation: A man is a wolf rather than a man to another man, when he hasn't yet found out what he's like.
Often quoted as "Homo homini lupus" [A man is a wolf to another man].