Definition from Wiktionary, the free dictionary
When English pubs closed in the afternoon, customers who were waiting or arrived soon after the pub re-opened in the evening were known as 'early doors'.
This etymology is incomplete. You can help Wiktionary by elaborating on the origins of this term.
- (Northern England) Early; at a time before expected; sooner than usual.
- (Cockney rhyming slang, plural only) woman's drawers.