From Middle English glopnen, from Old Norse glúpna (“to frighten, grieve, look downcast”), from Proto-Germanic*glupnōną (“to frighten, cause to stare”), from Proto-Indo-European*ghlub(h)- (“to yawn, gape”). Cognate with Icelandicglúpna (“to put to shame”). More at glope.
English Wiktionary. Available under CC-BY-SA license.