It’s an old Anglo-Saxon word with only one basic meaning, and there’s nothing special about it. It wandered quietly down the centuries from Old English, minding its own business. Maybe that’s how it accumulated so many possessions. There’s knock out, which has at least two meanings, neither of which is the opposite of knock in. There’s knock off, which has at least three meanings, none of which is the opposite of knock on. Then there’s knock up (two meanings in American, and more in the UK) and knock around and knock together (which may actually be the opposite of knock apart), and I bet you can think of some more.
Originally written 1994.
Added here April 2011.