You (pronoun of address) be (imperative verb of being) cool (at a comfortably mild temperature)
~or~
You (Pronoun subject) be (used, incorrectly, as the 2nd person present indicative) cool (oh, with an admirable serene personal style.)
And it doesn't avoid the ambiguity to say, "You be chillin'."
You an' all your chillen...
Good ... good ...
Um, what are you drinking?
..
You know, just askin’.