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.