E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
properly means one who has attained (from the Latin, adeptus, participle of adipiscor). The alchemists applied the term vere adeptus to those persons who professed to have attained to the knowledge of the elixir of life or of the philosophers stone.
Alchemists tell us there are always 11 adepts, neither more nor less. Like the sacred chickens of Compostella, of which there are only 2 and always 2a cock and a hen.