Is it ensure, insure, or assure?
early 15c., "grotto, cavern," from Latin crypta "vault, cavern," from Greek krypte (short for krypte kamara "hidden vault"), fem. of kryptos "hidden," verbal adjective from kryptein "to hide," from PIE *krau- "to conceal, hide" (cf. Old Church Slavonic kryjo, kryti "to hide"). Meaning "underground burial vault or chapel in a church" first attested 1789.
A small pit, recess, or glandular cavity in the body.