c. 1300, plural of Middle English galwe "gallows" (mid-13c.), from Old Norse galgi "gallows," or from Old English galga (Mercian), gealga (West Saxon) "gallows;" all from Proto-Germanic *galgon "pole" (source also of Old Frisian galga, Old Saxon galgo, Middle High German galge "gallows, cross," German Galgen "gallows," Gothic galga "cross"), from PIE *ghalgh- "branch, rod" (source also of Lithuanian žalga "pole, perch," Armenian dzalk "pole").
In Old English, also used of the cross of the crucifixion. Plural because made of two poles. Gallows-tree is Old English galg-treow. Gallows humor (1876) translates German Galgenhumor.