'sinus' originates from Latin, specifically the word 'sinus', where 'sinus' meant 'bay, fold, curve or hollow'.

'sinus' was used in Classical Latin with meanings like 'bay' or 'fold'; Medieval Latin and later medical Latin retained the term for bodily cavities, and it entered English as 'sinus' in Late Middle English (keeping the same form).

Initially, it meant 'bay, fold, curve or hollow' in a general geographical/figurative sense, but over time it came to be used especially for bodily cavities (and in specialized fields like anatomy and medicine) with the modern primary meaning of 'a cavity or channel in the body'.