Druid: a priest of the Celtic religion, often called Druidism.
From drui, Old Irish term of disputed origin. Later Latinized by
Julius Caesar as druis, plural druides. Possible derivation from
the Greek drus: oak, or from druwides:  dru-: superlative Indo-
European prefix, or deru- also dreu-, "solid, firm, made of
wood" -wid: Indo-European *weid-, "to see"; compare with videre
(Latin) and idein (Greek)--to see, to know. Hense, either all-
knowing, or knowers of the oak.  Or, from Indo-European  Modern
Gaelic has the word draoi: sorcerer, while modern Welsh has
derwydd: wizard.          © Webster 1913.