From Latin 'deponere' (to put down) — something placed down, whether money in a bank or sediment on a riverbed.
A sum of money placed in a bank account or given as security; a layer of matter laid down by a natural process; to put something down in a specific place or entrust it for safekeeping.
From Latin 'dēpositum' (something put down, a thing entrusted), the neuter past participle of 'dēpōnere' (to put down, to lay aside), from 'dē-' (down) and 'pōnere' (to put, to place). Unlike the verb 'depose,' which came through French '-poser,' 'deposit' was borrowed more directly from the Latin participial form, preserving the '-posit-' stem. The geological sense of natural deposits (mineral, sedimentary) developed in the 18th century. Key roots: pōnere / positum (Latin: "to put, to place"), dē- (Latin: "down, away from").
The French word 'dépôt' — which gave English 'depot' — is the same word as 'deposit,' just worn down by centuries of French sound change. The circumflex accent in 'dépôt' marks where the 's' of Latin 'dēpositum' was lost, a ghostly trace of the full Latin form that English 'deposit' still preserves.