It's a carpentry tool used to see if a piece is level. A buble inside a tube moves up in the direction of the tilt. This is a fairly typical spirit level:
Since the man was drunk, I think it wasn't really difficult to hit him - aren't such swords quite heavy and impractial when you don't know how to use them? (I don't know anything about swords etc, but it seems to me that a drunk man wouldn't be able to use it effectively).
Acctuly, they're rather light. In of them selves, they're some what heavy and un-weildly. But compare that to a Scottish claymore (4 ft long and weighing up to 10lb.s) they are the perfect weapon for a drunk guy who doesn't know how to use a sword. If he had been trained, the story might go very differntly.