sinusitis n.

early 15c., in anatomy, "hollow curve or cavity in the body" (Chauliac), from Medieval Latin sinus, from Latin sinus "bend, fold, curve, a bent surface; a bay, bight, gulf; a fold in land;" also "fold of the toga about the breast," hence "bosom," and figuratively "love, affection, intimacy; interior, inmost part;" a word of unknown origin. De Vaan writes that it is "probably to be connected with" Albanian gji "breast." In 17c.-18c. English writers also sometimes used it in the classical senses of "a bay, gulf, or arm of the sea; a cavity or hole in the earth," but these uses are obsolete.

word-forming element in medicine denoting "diseases characterized by inflammation" (of the specified part), Modern Latin, from Greek -itis, feminine of adjectival suffix -ites "pertaining to." Feminine because it was used with an implied nosos "disease," a feminine noun; especially in arthritis (nosos) "(disease) of the joints." Arthritis (16c.) was one of the earliest appearances of the suffix in English and from it the suffix was abstracted in other uses.