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" (cf. Old Frisian galga, Middle High German galge "gallows, cross," German Galgen "gallows," Gothic galga "cross"), from PIE *ghalgh- "branch, rod" (cf. Lithuanian zalga "pole, perch," Armenian dzalk "pole"). In Old English, also used of the cross of the crucifixion. Plural because made of two poles.
Heb. 'ets, meaning "a tree" (Esther 6:4), a post or gibbet. In Gen. 40:19 and Deut. 21:22 the word is rendered "tree."