Is it ensure, insure, or assure?
early 15c., from Old French concave (14c.) or directly from Latin concavus "hollow, arched, vaulted, curved," from com-, intensive prefix (see com-), + cavus "hollow" (see cave (n.)).
concave con·cave (kŏn-kāv', kŏn'kāv')adj. Curved like the inner surface of a sphere. n. A concave surface, structure, or line.