rule of thumb

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Meaning: an approximation, an extremely simplified version (often wrong), a
rule that is often (but NOT always) correct

Example: A grown man's thumb is almost one inch wide... But that depends on
how big of a hand you are, and how thick your fingers are. Thus, measuring
things by \"rule of thumb\" is rather inaccurate.

Origin: A lot of English measurement units are based on a Monarch's
personal body measurements. Foot, inch, yard, are all based off of a body
part, and standardized later.