An Etymological Dictionary of the German Language/greinen
greinen, vb., ‘to whine, grin,’ from the equiv. MidHG. grînen, OHG. grînan, str. vb., ‘to distort the mouth with laughing or crying, grumble, snarl,’ allied to MidHG. grinnen, ‘to gnash,’ E. to grin, to groan, (AS. grânian), also grinsen; from the OG. cognates Ital. digrignare, ‘to grin,’ is derived. The root grī̆, pre-Teut. ghrī̆, is not found elsewhere (Sans. hrî, ‘to be ashamed’?).