sup is short for wassup, which is short for what's up, which is short for what is up.
It originated from 2000s teen slang
Sometimes mistaken for "shut up"
it is an interjection, and is used as an informal greeting, such as "what's up?"
Sup is a gang associated word, meaning "I hate you and want you dead." Many think the word sup means hello or what's up. Using the word sup can be seen as offensive to gang members and using it the wrong area can end up getting you and your family killed.