I have heard it was a common law rule about the thickness of a switch with which no punishment would occur for spousal abuse. I have also heard that this is not correct.
I cannot find a definitive source Rule of thumb beat meaning. The suggestion appears to be of late 20th-cent. The site is blocked from work so I can't send you a direct link to the episode, but it's titled "The Bee's Knees" http: Searched for the origin of this one without success.
Always imagined it to be a naval reference to torpedo tubes because once it's gone down the tubes it isn't coming back.
I guess I always associated that with plumbing. Surely they call the plumbing "tubes" somewhere?