Why was "tantrum" trending last week?
Old English smearcian "to smile." No exact cognates in other languages, but probably related to smerian "to laugh at, scorn," from Proto-Germanic *smer-, *smar-, variant of PIE *smei- "to smile;" see smile (v.), which after c.1500 gradually restricted smirk to the unpleasant sense "smile affectedly; grin in a malicious or smug way." In some 18c. glossaries smirk is still simply "to smile." Related: Smirked; smirking. The noun is recorded by 1560s.
1550s, from smirk (v.).