The noun was initially used by alchemists (as you’ll read below), which I did not know. The etymology is from the sense of “attainment” rather than “skilled” which is not what I would have expected.
adept, a. and n.
A. adj. Completely versed (in); thoroughly proficient; well-skilled.
B. n. [In med.L. adeptus was used subst. and assumed by alchemists that professed to have attained the great secret. In Eng. the L. form was at first used, with pl. adepti.] Hence, ‘He that is completely skilled in all the secrets of his art.’ J. One that has attained to proficiency in anything.
Hence adeptly adv., in an adept manner; skilfully, adroitly.