Sup is a contraction or aphetic of the older term ''what's up?'', Does anyone know how it has originated?
what’s up? phr. Green's Dictionary of Slang The usages and dates also here. also ’sup? wassup? whassup? wha’s up? what up? whazzap? wuzzup? ’zup?
From what's up':
1855 [UK] W. Phillips Wild Tribes of London 104: Hilloh, Tom! what’s up, that you are obleeged to patch up your crab-shells? [...] Mr. Bradley [...] replies that ‘nuffin’s up’, but that times is bad.
1915 [UK] T. Burke Nights in Town 305: Some cried ‘Whassup?’.
used as a greeting US A slurred “what’s up?”. — Connie Eble (Editor), UNC-CH Campus Slang, p. 6, Fall 1981
this term of greeting has progressed from standard grammar to the shortened lazy colloquial.