sneer v.

1550s, "to snort" (intransitive, of horses), perhaps from North Frisian sneere "to scorn," related to Old English fnæran "snort, gnash one's teeth," and of imitative origin (compare Danish snærre "grin like a dog," Middle Dutch, Middle High German snarren "to rattle").

The meaning "grin or smile contemptuously" is from 1670s; that of "speak derisively, insinuate contempt in words" is by 1707; the sense of "to curl the upper lip in scorn" is attested from 1775. Related: Sneered; sneering. Sneer word is in William Safire's columns from 1980 (he lists so-called, self-proclaimed, would-be, and purported).

"derisive or contemptuous grin," also "verbal expression of contempt," 1707, from sneer (v.).