rule-of-thumb

The phrase "rule of thumb" is a long-standing English idiom with debated folk etymologies. The most commonly cited, though historically unverified, origin traces it to a practice among carpenters and other tradesmen who would use the width of their thumb (approximately one inch) as a rough, readily available unit of measurement. Another theory suggests it relates to an old English common law principle (also unproven) that permitted a man to beat his wife with a stick no thicker than his thumb. Regardless of its precise genesis, the phrase's logic evolved from the concept of using a part of the body—the thumb—as an imprecise but practical tool or guide. This concrete image gave rise to its modern abstract meaning: a broadly accurate, experience-based principle or heuristic method, as opposed to a strictly scientific or theoretical calculation.

This is a fixed idiomatic compound. It is not productively formed from separable modern morphemes for the purpose of this analysis. The individual words 'rule', 'of', and 'thumb' are simplex forms.