early 15c. variant of Middle English gnasten "to grind the teeth together" in rage, sorrow, or menace (early 14c.), perhaps from Old Norse gnasta, gnista "to gnash the teeth," of unknown origin, probably imitative. Compare German knistern "to crackle," Old English gnidan "to rub, bruise, pound, break to pieces," Danish knaske "crush with the teeth." Related: Gnashed; gnashing.
