A Chesea Dagger is a broken bottle used as a knife or stabbing weapon.
You hold for example a beer bottle at its neck and smash it into a chair or table so that the sharp edges can be used to stab someone.
Sometimes used by drunk footballfans therefor the name.
Mike got into a fight at the pub last night. He got stabbed three times by this london prick using a chelsea dagger!
A pun on "Britney Spears" used by a Burlesque dancer, and the inspiration for the smash Fratellis hit of the same name.
"Chelsea Dagger is a cracking tune!"