infirm

The word "infirm" is a direct borrowing from Latin infirmus, which itself is a compound of the negative prefix in- and the adjective firmus (strong, stable). The literal Latin meaning, "not strong," has been preserved almost unchanged into modern English. While firmus evolved into the English word "firm" with connotations of physical solidity and commercial stability, its pairing with the negative prefix created a specialized term primarily describing a lack of physical or mental strength, often associated with illness or old age. This etymological logic is transparent: the core concept of strength (firm) is negated (in-), resulting in the modern meaning of weakness or frailty.

Structure: in (not) + firm (strong) - in: A Latin prefix meaning "not, opposite of, without." It functions as a negative or privative prefix. - firm: From Latin firmus, meaning "strong, stable, steadfast." It functions as the core root conveying the concept of strength or stability.