balls-deep

Short definition from Wiktionary

Adverb

1. 1
   With the penis fully immersed in an orifice, more often the anus or vagina than the mouth.
2. 2
   Extremely; to the fullest extent possible.

Adjective

1. 1
   Having the penis fully immersed in an orifice, more often the anus or vagina than the mouth.
2. 2
   Fully committed to a situation.