It's a verb (and a bit of an archaic one).
sup is not a word
Sup
lysosomes sup if you go to newpoint
sup sup
The part of speech for this particular word is a noun.
part of speech
Assuming you mean sup as in "what's up?", it's spelled 'sup.
The part of speech for "answer" is a noun.
adverb
what part of speech is beneath
when happy say sup
The part of speech for "unfamiliar" is an adjective.