Usually preceded by “a little” as in “If you could maybe slip me a little somethin’ somethin’” and an implicit or explicit wink.
I’m drawing more of a blank than I’d expect to w/regards to where I’ve heard it recently, although Olive utilized it in Pushing Daisies… anyway, it’s around and out there, but from where?
More properly, it’s “sumpin’ sumpin’.” I suspect its origin is in Africo-American vernacular, possibly hip-hop, from the 1990s. Anyway, I recall first hearing in in the late 90s, and I’m usually late to the party.
If you’re looking for a first citation, though, the earlies I could find in a minute or so of Googling was 1994. But, hey, it’s a free bump until a real researcher can answer.
I recall Spongebob Squarepants using it in either an episode or The Spongebob Squarepants Movie. Don’t remember the context, however.