Definition of balls deep in English:

balls deep

adjective

vulgar slang US
  • Thoroughly immersed or involved.

    • ‘I'm balls deep in the music scene’
    • ‘the United States would be balls deep in military conflicts’

adverb

vulgar slang US
  • To a considerable level of immersion or involvement.

    • ‘I needed something to keep me busy and just went balls deep into fantasy baseball’

Origin

1990s from balls + deep, probably as a sexual metaphor.