sinuation 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.