BBIMF is one of many, many, many acronyms used at TFINO.
"BBIMF" is an acronym for "Big Boobies In My Face," an expression of positive reaction. Because big boobies in your face is awesome.
While Cuban Pete introduced this phrase to TFINO, he is not the originator of it. It was first coined in the early 90's by one of Cuban Pete's fraternity brothers.