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I believe 5. refers to "rule of thumb" in its supposed sense that a man was permitted to beat his wife with a stick no wider than his thumb. This is almost certainly apocryphal, though often cited!
"Rule of thumb"'s origin is probably from using the thumb as a rough measure in carpentry, etc. This usage goes back centuries, and is still fairly common.
Last edited by Shayne Parkinson; 01-22-2014 at 05:29 PM.
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