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OK. I'm not sure if this is the origin, but here is my recollection of the phrase:
There used to be a TV show in the 60s called "The Dick Vandyke Show" starring, of course - Dick Vandyke.
For those of you younger folks, he was a very skinny guy who played something of a wimp and a giant klutz.
The show always opened with him tripping over something.

In one episode, he took a class in Karate.
After completing his course, he came home and told his wife: "These hands are now registered as lethal weapons."
Picture the skinniest, wimpiest, klutziest guy you know saying that and you will understand why it was so funny I remember it all these years later.