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.
"one of the order of priests among the ancient Celts of Gaul, Britain, and Ireland," 1560s, from French druide (16c.), from Latin druis, fem. druias (plural druidae), from Gaulish Druides, from Celtic compound *dru-wid- "strong seer," from Old Celtic *derwos "true" (from PIE root *deru- "tree," especially oak) + *wid- "to know" (from PIE root *weid- "to see"). Hence, literally, perhaps, "they who know the oak" (perhaps in allusion to divination from mistletoe). Anglo-Saxon, too, used identical words to mean "tree" and "truth" (treow).
The English form comes via Latin, not immediately from Celtic. Old English had dry "magician," presumably from Old Irish drui. The Old Irish form was drui (dative and accusative druid; plural druad), yielding Modern Irish and Gaelic draoi, genitive druadh "magician, sorcerer." Not to be confused with the United Ancient Order of Druids, a secret benefit society founded in London 1781.
Old English treo, treow "tree" (also "timber, wood, beam, log, stake"), from Proto-Germanic *trewam (source also of Old Frisian tre, Old Saxon trio, Old Norse tre, Gothic triu "tree"), from PIE *drew-o-, suffixed variant form of root *deru- "be firm, solid, steadfast," with specialized senses "wood, tree" and derivatives referring to objects made of wood.
The line which divides trees from shrubs is largely arbitrary, and dependent upon habit rather than size, the tree having a single trunk usually unbranched for some distance above the ground, while a shrub has usually several stems from the same root and each without a proper trunk. [Century Dictionary]
The widespread use of words originally meaning "oak" in the sense "tree" probably reflects the importance of the oak to ancient Indo-Europeans. In Old English and Middle English also "thing made of wood," especially the cross of the Crucifixion and a gallows (such as Tyburn tree, famous gallows outside London). Middle English also had plural treen, adjective treen (Old English treowen "of a tree, wooden"). For Dutch boom, German Baum, the usual words for "tree," see beam (n.). Meaning "framework of a saddle" is from 1530s. Meaning "representation of familial relationships in the form of a tree" is from c. 1300. Tree-hugger, contemptuous for "environmentalist" is attested by 1989.
Minc'd Pyes do not grow upon every tree,
But search the Ovens for them, and there they be.
["Poor Robin," Almanack, 1669]