From the archives:

Mental, in anatomy, is an adjective meaning "related to the chin". It comes from the Latin word for chin, mentum.

Its use is seen in the mentalis, a muscle covering the tip of the chin, and the mental foramen, a small passageway found near the chin on each side of the mandible through which the mental artery, vein, and nerve pass.

Across all cultures it is common for people to touch their their chins as they ponder, but mental as in "mental images" comes from mens, Latin for mind, and not from mentum. However, as mentum is so closely linked to mens through our behavior, some etymologists believe (albeit without evidence) that both words are somehow etymologically related.

The Thinker by Auguste Rodin: engaged
in two types of mental activity.