Definition of balls deep in US English:

balls deep

adjective

US
vulgar slang
  • Thoroughly immersed or involved.

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

adverb

US
vulgar slang
  • 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.