E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A chief priest (Celtic, der, superior; wydd, priest or instructor). In Taliesin we read, Bûm gwŷdd yngwarth an (at length I became a priest or wydd). It was after this period that the wydds were divided into two classes, the Derwydds and the Go-wydds (Druids and Ovidds). Every chief had his druid, and every chief druid was allowed a guard of thirty men (Strabo). The order was very wealthy. (Not derived from the Greek drus, an oak.)
Patricius tells us that the Druids were wont to borrow money to be repaid in the life to come. His words are, Druidæ pecuniam mutuo accipiebant in posteriore vita reddituri.