The modern word druid comes from the Latin druides, but behind that Latin word is Old Irish, Old Cornish and Middle Welsh words which hypothetically might be based on a proto-Celtic word reconstructed as druwids (plural is druwides). Druid is thought to come from the Celtic word for the oak tree, duir. A drewid is a “knower of oak trees”.
What led me to this clearance of understanding was a look at one of Joseph Beuys’s works (1982) which consisted of the planting of 7000 oak trees in Kassel in Germany. in conversation with Richard Demarco, Beuys said:
I think the tree is an element of regeneration which in itself is a concept of time. The oak is especially so because it is a slowly growing tree with a kind of really solid heart wood. It has always been a form of sculpture, a symbol for this planet ever since the Druids, who are called after the oak. Druid means oak. They used their oaks to define their holy places. I can see such a use for the future … The tree planting enterprise provides a very simple but radical possibility for this when we start with the seven thousand oaks.
Other words derived from this root (excuse pun) include the Old English treow from which we have tree, truce, truth, troth, tryst – what a vast array of fruit those words represent! And that leads me to the moment Jesus was hung from the remains of a felled tree and, with true love, excruciatingly transformed the Tree of Death to the Tree of Life.
PS You may be interested in a series of poems written by Jim Bridgman called The Tree Cycle, for example, this Nightmare of the Rood