The American Heritage Dictionary fifth edition s.v. druid offers:
A member of an order of priests in ancient Gaul and Britain who appear in Welsh and Irish legend as prophets and sorcerers.
[From Latin druidēs, druids, of Celtic origin; see deru- in the Appendix of Indo-European roots.]
The druids were members of a priestly social class in ancient Celtic Gaul (now much of the continent of Europe, especially France) Ireland, and Britain. The word “druid,” or rather druides, is first attested in Latin and Greek, though it is of Celtic origin. Irish forms of drui (singular) and druid occur fairly frequently in Medieval Irish literature, though mostly as references made in passing, and much more often than the Welsh cognate drywoccurs in early Welsh texts.
What is the Etymology of Druid?
In Indo-European terms, according to the 2015 Fifth Edition of The American Heritage Dictionary IE Appendix edited by Calvert Watkins, the proto-Celtic form *dru-wid or strong seeing, is formed from the Proto I. E. root *deru “strong” and *ˆwid- “to see.” Druid then literally means “strong see-er.”
Early etymologies sometimes translate *dru-wid/druid as “oak seeing.” This is a reasonable error; I. E *dru also gives us the word oak because oak is a strong wood, known even now for its durability. Modern English “tree,” “trencher” and “trough” are all also derived from I. E. *deru because they are made out of wood, a strong substance. *Weid– “to see” also gives us modern English “video” and “wise.”
What Did Druids Do?
There are a number of other words associated with the functions performed by members of the druid class and with the social roles of the druid class in the various languages associated with the Celts. Irish fili, plural filid is usually translated as “poet,” which is not unreasonable, though the fili also had other functions. Filid comes from the same Celtic root as the Welsh word gweled “see,” and it seems that one of the functions of the filid was that they were seers (Williams and Ford 1992, 21). Fáith Irish for prophet (plural fátha) is often used interchangeably with filid.
The Uraichech Becc a medieval Irish law tract, describes fáithsine or “prophecy” as the function of the fili. The general interpretation made by most Celticists is that the fili and the fáith were originally a single class, and one that was closely related to the druid class (Williams and Ford 1992, 22). The Irish fili and the fáith would an Irish equivalent of the class called ovate by Greek authors, and vates by the Romans (most notably, Strabo); the words are etymologically related.
Vates is cognate with Welsh gwawd, a word that used to mean “song” but gradually evolved to mean “satire.” It seems reasonable then to conclude that the vates would be present as “seers” at a sacrifice at which the druids would officiate as priests; this would explain some of the contradictory confusion between the druides and vates in Classical authors (Williams and Ford 1992, 22).
According to Uraichech Becc the fili was of a higher social status than the druid. The filid are classed with the lords, while the druids are classed with craftsmen like smiths and other artisans. This may well reflect a later state of affairs; I think it does, after the decline of the druid class with the introduction of Christianity. It appears that the filid began to take over some of the druid functions and social prestige as the druids declined in power. I would even argue that the Irish jurist, the brithem (the brehon) was also part of the fili class.
The lowest of the three groups in social status are the baird, a lower order of poets, called by the Greeks (via reference to the Gaulish varieties) bardoi and by the Romans bardi. The bards too may have suffered from the increasing stature of the filid(Williams and Ford 1992, 23). The bards have a lower honor price in the law tracts (half that of the filid).
We see the Morrígan functioning as a fátha in the medieval Irish epic The Táin. She predicts the future, and delivers her prediction in the form of poetry.
Williams, J. F. Caerwyn. Irish Literary History. Trans. Patrick K. Ford. University of Wales Press, Cardiff, Wales, and Ford and Bailie, Belmont, Massachusetts. Welsh edition 1958, English translation 1992.