Is it ensure, insure, or assure?
late 13c., from Anglo-French tumbe, Old French tombe (12c.), from Late Latin tumba (cf. Italian tomba, French tombe, Spanish tumba), from Greek tymbos "burial mound, grave, tomb," from PIE root *teu- "to swell" (see thigh). The final -b began to be silent 14c. (cf. lamb, dumb). The Tombs, slang for "New York City prison" is recorded from 1840.