Is it ensure, insure, or assure?
late 14c., "to hold to" (a manner of life or course of conduct), from Old French observer, osserver "to observe, watch over, follow" (10c.), from Latin observare "watch over, note, heed, look to, attend to, guard, regard, comply with," from ob "over" (see ob-) + servare "to watch, keep safe," from PIE root *ser- "to protect." Meaning "to attend to in practice, to keep, follow" is attested from late 14c. Sense of "watch, perceive, notice" is 1560s, via notion of "see and note omens." Meaning "to say by way of remark" is from c.1600. Related: Observed; observing.