Is it ensure, insure, or assure?
"hollow curve or cavity in the body," early 15c., 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;" of unknown origin.
sinus si·nus (sī'nəs)
A depression or cavity formed by a bending or curving.
A channel for the passage of blood or lymph, without the coats of an ordinary vessel, such as the blood passages in the gravid uterus or in the cerebral meninges.
Any of various air-filled cavities in the bones of the skull, especially one communicating with the nostrils.
A dilatation in a blood vessel.
A fistula or tract leading to a pus-filled cavity.