- Expression meaning to be selfish or unfair to your mates.
- Slang for Royal Navy Sailor. In the RN it means someone who's just a little too happy to be in the Navy - white ensign out of their window, anchor tatoo, all that sort of thing.
- Originally from the 50's film line "I'm alright jack, how about you?"
- Potential alternative origin, possibly what led to the above usage: from the slang 'on (your) Jack Jones', meaning 'on your own'; mockney rhyming slang or some such. Someone looking after his 'Jack Jones', looks out only for himself, hence 'Jack'.
- Medieval body armour