The word Cowan entered wiccan circles though Freemasonry.
From Albert G. Mackey's (1908) Lexicon of Freemasonry:
Cowan: One of the profane. This purely masonic term is derived from the Greek kuon, a dog. In the early ages of the church, when the mysteries of religion were communicated only to initiates under the veil of secrecy, the infidels and unbaptized profane were called "dogs," a term probably suggested by such passages of Scripture as Matt. vii 6, "Give not that which is holy unto the dogs," and Philip, iii. 2, "Beware of dogs, beware of evil workers, beware of the concision." Hence, as kuon, or dog, meant among the early fathers one who had not been initiated into the Christian mysteries, the term was borrowed by the Freemasons, and in time corrupted into cowan. The attempt made by some anti-masonic writers to derive the word from the chouans of the French Revolution is absurd. The word was in use long before the French Revolution was even meditated. I have in my possession a copy of the edition of Anderson's Constitutions, printed in 1769, which contains at p. 97, this word: "Working Masons ever will have their own wages *** let cowans do as they please."
Another interpretation of this term as a result of later investigation proves it to be a Sone Mason capable of building only dry walls.