“Rule of thumb”
What it means
A rough practical method, good enough without precision.
Where it comes from
The thumb has served as an approximate inch across many cultures, and craftsmen without a rule to hand measured by it routinely. Dutch and several Scandinavian languages use the same word for thumb and inch.
How sure are we?
The thumb-as-inch explanation is plausible and fits the linguistic evidence: Dutch, Swedish and French all use one word for thumb and inch. It is also demonstrable, since the measure is still in use — the Royal Navy teaches a rule of thumb in damage control shoring, measured from thumbnail to hand, to set the angle of a shore. The widely repeated claim that the phrase refers to a legal maximum thickness of stick for beating a wife has no basis: no such law existed, and the story began as eighteenth-century satire before being taken literally.