sinusoid (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 meaning "like, like that of, thing like a ______," from Latinized form of Greek -oeidēs (three syllables), from eidos "form," related to idein "to see," eidenai "to know;" literally "to see" (from PIE *weid-es-, from root *weid- "to see"). The -o- is connective or a stem vowel from the previous element. Often implying an incomplete or imperfect resemblance to the thing indicated.