Is it ensure, insure, or assure?
late 15c., "unlucky," from Old French senestral, sinistral or Medieval Latin *sinistralis, from sinister (see sinister). Meaning "on the left side" is from 1803. Related: Sinistrally.
sinistral sin·is·tral (sĭn'ĭ-strəl, sĭ-nĭs'trəl)adj.
Of, facing, or located on the left side; left.