Dru-wid" combines the word roots "oak" and "knowledge
The word "Druidae" is of Celtic origin. The Roman writer Pliny the Elder (Gaius Plinius Secundus, 23/24-79 A.D.) believed it to be a cognate with the Greek work "drus," meaning "an oak." "Dru-wid" combines the word roots "oak" and "knowledge" ("wid" means "to know" or "to see" - as in the Sanskrit "vid").
When they talk about 'Oak Knowledge' remember as people migrated to the frozen north the flora and fuana changed. Where they may have had 'Palm Tree' or Sycamore or Acacia knowledge in Egypt as they migrated to a place above the equator and the indigenous trees were few and far between - over time they used another tree icon like the Oak d Yew or Evergreens as the symbol of wisdom.
'The oak (together with the rowan and hazel) was an important sacred tree to the Druids. In the Celtic social system, Druid was a title given to learned men and women possessing "oak knowledge" (or "oak wisdom").'
The Druids were, is the Gaelic word 'Druidh', describing a wise man, or the holder of much secret knowledge. The wisdom these priests were privy to consisted of healing potions - as in herbalism, they were teachers, legislators, astronomers, astrologers, chemists, musicians, poets, the holders of all religious holy days and rituals, philosophers, and judges.