The Spanish word 'frenillo' comes from the Latin word 'frenum', which meant 'bridle' or 'horse bit' - a device used to control horses. In Spanish, this evolved into 'freno' meaning 'brake' or 'restraint', and then the diminutive suffix '-illo' was added to create 'frenillo', literally meaning 'small restraint'. This perfectly describes its anatomical meaning as the small band of tissue that restrains or connects parts of the body (like the one under the tongue).

A simpler related Spanish word is 'freno', which means 'brake' or 'restraint'. This connection makes intuitive sense since both a brake and a frenillo serve to restrain or hold something back - a brake restrains a vehicle's movement, while a frenillo restrains tissue in the body.

The English medical term 'frenum' (or 'frenulum') comes from the same Latin root 'frenum'. Just like in Spanish, it refers to a small fold of tissue that restrains an organ's movement, such as the one under the tongue or upper lip. The connection to the original meaning of 'bridle' or 'restraint' is clear, as these anatomical structures act like tiny natural restraints in the body.