means A hollow cavity within a bone or body part — most familiarly the air-filled spaces in the skull around the nose, but also any pouch or channel, like the sinuses of the heart or veins.

from From Latin 'sinus,' meaning a bend, fold, curve, or hollow — originally the fold of a toga draped across the chest, hence a 'bosom' or pocket. Anatomists borrowed it for the body's curving cavities and pouches. The same word, in its mathematical journey, gave us 'sine': Arabic translators rendered a Sanskrit term for 'bowstring' as 'jiba,' which got misread as 'jaib' (bosom, fold), which Latin scholars dutifully translated back as 'sinus' — so your aching head and your trig homework share an ancestor.