"grin mockingly," c. 1400, perhaps from Scandinavian (compare dialectal Norwegian flira "giggle, laugh at nothing," dialectal Danish flire "to grin, sneer, titter"). Transitive sense from 1620s. Related: Fleered; fleering; fleeringly. As a noun from c. 1600.
fleer (n.)
"one who flees," late 14c., agent noun from flee (v.).