Definition of on one's jack (or Jack Jones) in English:

on one's jack (or Jack Jones)

phrase

British
rhyming slang
  • On one's own.

    • ‘The last time was 2002, when I played the Pyramid Stage on my jack jones, which was the most terrifying moment I've ever had on stage.’
    • ‘Two weeks later, it's a chilly October night, I'm on my Jack Jones and the ‘phone rings.’