Rule-of-thumb Definition

rules-of-thumb
noun
A rule based on experience or practice rather than on scientific knowledge.
Webster's New World
A useful principle having wide application but not intended to be strictly accurate or reliable in every situation.
American Heritage
Any method of estimating that is practical though not precise.
Webster's New World

(usually hyphenated) Approximated, guesstimated.

I made a quick, rule-of-thumb estimate of the manhours required for the job.
Wiktionary
The definition of a rule of thumb is a generally accepted guideline, policy or method of doing something based on practice rather than facts.
An example of a rule of thumb is the general guideline that you don't wear white after Labor Day.
YourDictionary
Synonyms:
guidelineguidepostgeneral guidelineapproximationestimate
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Other Word Forms of Rule-of-thumb

Noun

Singular:
rule-of-thumb
Plural:
rules-of-thumb

Origin of Rule-of-thumb

  • Michael Quinion lists the first documented use as 1692 while The Oxford English Dictionary puts the first documented use at 1685. Some suggested origins include the fact that the inch originated as the distance between the base of the thumbnail and the first joint, the practice of approximating the general direction of the wind by wetting the thumb then raising it in the air, and the rule of English Royal banquet plate setters using the distance of the thumb to equally space each plate from the table edge. Apocryphally, it has been claimed the term originally referred to the maximum thickness of a stick with which it was permissible for a man to beat his wife, but the earliest use in direct reference to domestic violence may be Del Martin's 1976 book Battered Wives.

    From Wiktionary

  • From the use of the thumb as a makeshift ruler or measuring device, as in carpentry

    From American Heritage Dictionary of the English Language, 5th Edition

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