late 14c., deposicion, "dethronement, a putting down of a person from dignity, office, or authority," from Old French deposicion (12c.), from Latin depositionem (nominative depositio), noun of action from past-participle stem of deponere "to lay aside" (see deposit (v.)).
Meaning "a statement or statements made in court under oath" is from early 15c. Meaning "action of depositing" is from 1590s. Properly, deposition belongs to deposit, but deposit and depose have become inextricably confused and English deposition partakes of senses belonging to both.