1. The date by which a perishable item must be sold by a store. Did you check the sell-by date on that carton of milk?

2. slang By extension, the figurative date by which one's usefulness or skill ends, usually due to aging. Primarily heard in UK. I'm sure they think I'm past my sell-by date after I mixed up all those important files. We need to find a new lead dancer, as Bella is past her sell-by date now and only getting older.