More specifically, it's a phrase used to describe the extremely hardcore state of mind a player is in when he uses the "All Out Attack" offensive strategy in NHL 2004/2005. It is usually used as a desperation strategy when you're getting your butt kicked by your friends.
Player 2: Oh no, he's goin' balls deep!
2) (adj) generally, the act of doing any activity hardcore, all out, and as fully as possible.
A: Balls deep in some slut's filth.
ex 2. We hit the bars balls deep last night- I feel shitty.
2.can be used as an expression for something overwhelming, ridiculous, exaggerated, or to refer to a large number of things. Also as an affirmative when answering a question
"Are you in to go to tijuana tonight? IM BALLS DEEP"