Although it probably makes you think more of snakes than head colds, sinuous is etymologically more like sinus than serpent. Sinuous and sinus both derive from the Latin noun sinus, which means "curve, fold, or hollow." Other sinus descendants include insinuate ("to impart or suggest in an artful or indirect way") and two terms you might remember from math class: sine and cosine. In English, sinus is the oldest of these words; it entered the language in the 1400s. Insinuate appeared next, in the early 1500s, and was followed by sinuous and sine in the latter half of the 1500s, and cosine in the 1600s. Serpent, by the way, entered English in the 13th century and comes from the Latin verb serpere, meaning "to creep."