An Etymological Dictionary of the German Language/Annotated/greinen
greinen, verb, ‘to whine, grin,’ from the equivalent Middle High German grînen, Old High German grînan, strong verb, ‘to distort the mouth with laughing or crying, grumble, snarl,’ allied to Middle High German grinnen, ‘to gnash,’ English to grin, to groan, (Anglo-Saxon grânian), also grinsen; from the Old German cognates Italian digrignare, ‘to grin,’ is derived. The root grī̆, pre-Teutonic ghrī̆, is not found elsewhere (Sanscrit hrî, ‘to be ashamed’?).