accessory n.

also accessary, early 15c., "that which is subordinate to something else," also as a legal term, "one aiding in a felony without committing the offense" (as by advising, inciting, concealing), from Late Latin accessorius, from Latin accessor, agent noun of accedere "to approach" (see accede).

Strictly the noun (a person) should be accessary, the adj. (and noun, a thing) accessory; but the distinction is too fine to be maintained. [Century Dictionary]

Especially in the visual arts, "object introduced to balance composition or enhance artistic effect" (1540s). Attested from 1896 as "woman's smaller articles of dress;" hence accessorize. Related: Accessorial.

1550s, "subordinate;" c. 1600, "aiding in crime;" 1610s, "aiding in producing some effect," from Late Latin accessorius, from accessor, agent noun from accedere "to approach" (see accede). Meaning "aiding in crime" is from c. 1600.