Drew Mackie of Back of the Cereal Box has commented once or twice on my posts on Indo-European words that have traveled far afield. So here's another!
druid is from Latin druidæ, druides from Proto-Celtic *druwid- "priest". (Compare Irish Gaelic draoi, druadh, Scots Gaelic draoi, draoidh, druidh.) This is probably formed from the Proto-Indo-European roots *deru- "to be firm, solid, steadfast" (the source of tree and true) and *weid- "to see". So the etymological sense of druid is either "true seer" or "oak, tree + seer", druidical rites being associated with the oak.
*weid- in the form *woid-o- became Sanskrit वेदः vedaḥ "knowledge". The Rigveda (ऋग्वेद r̥gveda from ऋच् r̥c "praise, verse" from PIE *erkʷ- "to radiate, beam, praise", plus vedaḥ) is an ancient collection of hymns dedicated to the gods. The Rigveda dates to 1000 BC and is our first record of Indic.
*weid- also became Old English wīs as in wise, Greek ἰδέα "appearance, form, idea" as in idea, and Latin vidēre "to see" as in video, vision.