Is it ensure, insure, or assure?
1550s, "to snort" (of horses), perhaps from North Frisian sneere "to scorn," related to Old English fnæran "to snort, gnash one's teeth," of imitative origin (cf. Danish snærre "to grin like a dog," Middle Dutch, Middle High German snarren "to rattle"). Meaning "to smile contemptuously" is from 1670s; sense of "to curl the upper lip in scorn" is attested from 1775. Related: Sneered; sneering. Sneer word is in E. Digby Baltzell (1987).
1707, from sneer (v.).