Meanings
Attested since 1658. Of uncertain origin, but probably based on the use of the thumb as reference for various measurements. For instance, one theory notes that the inch originated as the distance between the base of the thumbnail and the first joint, another notes the practice of approximating the general direction of the wind by wetting the thumb then raising it in the air. A third theory notes that English royal banquet plate setters used the distance of their thumbs to equally space each plate from the table edge.
How to pronounce "rule of thumb":
A general guideline, rather than a strict rule; an approximate measure or means of reckoning based on experience or common knowledge.
Example: "The usual rule of thumb says that to calculate when an investment will double, divide 70 by the interest rate."
Approximated, guesstimated.
Example: "I made a quick, rule-of-thumb estimate of the manhours required for the job."
