[This is a fast-access FAQ excerpt.]
Wicca is "a pagan nature religion having is roots in pre-
Christian Europe and undergoing a 20th-century revival" (AHD3).
Only the most recently published dictionaries contain an entry for
it; RHUD2 dates it 1975. "Wicca" is a revival of an Old English
word that you can find in older dictionaries by looking up the
etymology of either "witch" or "wicked". In Old English, wicca
was the masculine form of a word meaning "wizard" or "sorcerer".
(The feminine form was wicce. "Witch" comes from wicce.)
Wicca and wicce came from from a proto-Germanic (not Celtic)
wikkjak, "one who wakes the dead", the first element of which
comes from the same Indo-European root as "wake".
Yes, we've heard the joke about the Beatles song "Wiccan, Work It