It's actually self explanatory.. This phrase, first popularized in the song Mellow by the artist HXBIBTI, means to pretend to be "chill" or "cool" (when you really just want to put yourself out there) just for the sake of a varying numerous reasons. (Ex. for a friend, or even a significant other.) Another variation of this phrase: "actin the mellow"
by GOAT LYRICS April 18, 2017