Gnash \Gnash\, v. t. [imp. & p. p. {Gnashed}; p. pr. & vb. n. {Gnashing}.] [OE. gnasten, gnaisten, cf. Icel. gnastan a gnashing, gn?sta to gnash, Dan. knaske, Sw. gnissla, D. knarsen, G. knirschen.] To strike together, as in anger or pain; as, to gnash the teeth.