Metaphor: the events of being deeply involved with something, this could be your homework; or something which has nothing to do with testicles or the length of orifices.
a. hey man what's happening?
b. im balls deep in my homework right now
a. that party last week was nuts
b. yeah! it was balls deep!
a. hear about that volcano eruption
b. yeah it totally wreeck
ed my neighborhood
c. eh, I was balls deep in the lava