Yours, Etc.: Origins and Uses of 8 Sign-Offs
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.).