Is it ensure, insure, or assure?
early 15c., variant of Middle English gnasten "to gnash the teeth" (c.1300), perhaps from Old Norse gnastan "a gnashing," of unknown origin, probably imitative. Cf. German knistern "to crackle." Related: Gnashed; gnashing.
Heb. harak, meaning "to grate the teeth", (Job 16:9; Ps. 112:10; Lam. 2:16), denotes rage or sorrow. (See also Acts 7:54; Mark 9:18.)