Similar to the rule of thumb. The rule of thumb was that it was legal for men to beat their wives as long as they used a stick no wider than their thumb. Some women take offense to the use of this phrase. Rule of wrist is much more appropriate for these nit-picking femanazis.
The rule of wrist here is to measure out an arms length.
Popularized by the film "The Boondock Saints", The Rule of Wrist is a fitting male translation of the original rule, "Rule of Thumb", which is; a man can beat his wife as long as the stick was no wider than your thumb, after drunken review, it was decided that a stick as wide as your thumb cant lay out proper punishment and the width of your wrist should be used instead. upon mentioning this to a female in a serious tone of voice, expect to be kicked in the testicles.
"Cant do much damage with that now can we? Prehaps it should have been Rule of Wrist?"