on (one's) Jack(redirected from on his Jack)
on (one's) Jack
old-fashioned Alone, abandoned, or isolated; on one's own. Primarily heard in UK. Coming home from the war, many veterans felt as though they were on their Jack as they tried to re-enter civilian life.
on your Jackon your own. British informal
This an abbreviation of the rhyming slang expression on your Jack Jones .