Yours, Etc.: Origins and Uses of 8 Sign-Offs
late 14c., "dethronement, putting down from dignity or authority," from Old French deposicion (12c.), from Latin depositionem (nominative depositio), noun of action from past participle stem of deponere (see deposit (v.)).
Meaning "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 totally confused and English deposition partakes of senses belonging to both.