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!
Desperate nerd with a tiny cock: I'm BALLSDEEP!!!!! If you can't feel this, I'm just gonna give up and go play Halo Reach!!!!!! Fuck sex!
Balls Deep is a term that was coined by my brother Dan and Im proud to say I use it atleast once a day
Man...That chick I met lastnight, I took her back to my place and went balls deep on that shit till the sun came up.