a feeb is a word used to describe someone you cannot believe actually exists. like, they're so simple minded and stupid that they are literally a side character in their own life who has a select amount of voicelines to switch between. the act of feebing is saying / doing something unnatural or silly.
some dude : dude do you know jerome ? he's such a feeb.
some other dude : i know right, he was feebing so hard in class !!