The word "sup" is a slightly-archaic term meaning to eat in small swallows, but perhaps it's an abbreviation for "superior," "supervisor," or "supremum."
Tell us where you found the word and the sentence in which it appears.
'Sup? (with an apostrophe) may be an abbreviated form of What's up?, but it's hard to be sure without seeing the context.
"Sup" as a verb can also mean "have supper":
Behold, I stand at the door, and knock: if any man hear my voice, and open the door, I will come in to him, and will sup with him, and he with me.—Revelation 3:20 (KJV)