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1.
one of the most commonly misconstrued sayings, an example used in Boondock Saints "a man is allowed to beat his wife with a stick no thicker than his thumb." A claim made by a satirical caricaturist by the name of James Gillray against a judge. The first and original use of the saying is as simple as the words. The thumb was used as a readily available tool of measuring. Such as the Tailor's rule of thumb "the circumference of the thumb twice, is the circumference of the wrist, twice the circumference of the wrist is the circumference of the neck, and then twice the circumference of the neck is the waist's circumference." Then there is the Brewer's rule of thumb where if the temperature of the wort wasn't insufferable by the thumb it was cool enough to pitch the yeast. It has now been used as a term of commonplace knowledge in a field.
"The rule of thumb in making a sale is confidence, without confidence you don't make a sale"
 
2.
During coitus, if you're ready to move from the pink to the stink, but you're not sure if she's into it, you can always gently mash your strongest thumb into her butthole, as if to say, "What do you think?" If she smiles or goes with it, trade your thumb for your cock Indiana Jones style.
SHORT ROUND: Dr. Jones, is that your thumb!
DR JONES: No... not anymore according to the rule of thumb.
by Strangling Jack April 30, 2010
 
3.
The rule that basically a lot of modern women will allow a man to sneak a finger up their ass during sex, but will draw the line at full anal sex. The rule of thumb states that a thumb, being thicker than the average finger, inserted into a female's ass consensually will ultimately then mean that the consenting female will allow said male to insert his penis into her ass.
Davy and Stuart were together last night, do you know what happened? No, but apparently she failed the rule of thumb.

Davy said he's meeting Stuart tonight and won't try anything until he's found out whether Stuart passes the rule of thumb.
by PhilMujar June 02, 2014
 
4.
Whatever the captain says goes.
For however long he is the captain it will be known as the rule of thumb
The Captain said we all need to work harder, and whatever the captain says goes. That's the rule of Thumb
by docspock July 29, 2014
 
5.
Commonly used term orginating in the early 1900s, when it was legal for men to beat their wives, as long as they used a stick no wider than their thumb.
Rule of thumb? Can't do too much damage with that thing, can we? Perhaps it should have been the Rule of Wrist. - The Boondock Saints
by YousaywhyIsaywhynot October 23, 2006
 
6.
This is the highest acceptable level to wear your pants. place your hand sideways across your lower stomach with the top of your thumb even with the bottom of your belly button. the top of your pants should be LOWER than the bottom of your hand.
look at that guy he needs to loosen that braided belt and use the rule of thumb.
by THEKosheerDillWhole May 14, 2009
 
7.
A rule from the old days, in which a man could beat a woman with something no bigger than his thumb. It is now used in common language, and most people don't even realise the origin.
According to rule of thumb, you must be this tall...
by Spiff C May 28, 2006