Old English grislic "horrible, dreadful," from root of grisan "to shudder, fear," with cognates in Old Frisian grislik "horrible," Middle Dutch grisen "to shudder," Dutch griezelen, German grausen "to shudder, fear," Old High German grisenlik "horrible;" of unknown origin; Watkins connects it with the PIE root *ghrei- "to rub," on notion of "to grate on the mind." See also gruesome, to which it probably is connected in some way.