Slang for "the shit." Possibly derived from censored music where the word "shit" was reversed and sounds like "ish." Now, people often self-censor and use the word "ish" in conversations where traditional cursing is frowned upon.
This new song is the ish, son.
That catch in the 4th quarter was the ish.
Your Mom's Sour Dieselwax is the ish, yo!