deponi

It is not de plus poni, because poni is not a word. The trap here is that both halves look like Esperanto. De is a real preposition and a real prefix, and the language does build verbs that way. But there is no root pon- in Esperanto, so deponi cannot be assembled: it arrives from Latin deponere as a single unit, exactly as inspiri arrives from inspirare with its own in- already fossilised inside it. The test is the same one every time. Remove the supposed prefix and see whether what is left is a word.

A useful consequence. Because the Latin joinery is invisible to Esperanto, deponi does not inherit any of the other things deponere went on to mean in English. It is not depose a king, which is senigi je la trono or detronigi. It is not the legal depose either. Deponi is to lay something down in somebody else's keeping.

The banking family. Deponejo is a depository or a left-luggage office, deponaĵo is the deposit itself, and deponanto is the depositor. Note the fence against ŝpari, which is to save in the sense of not spending: you ŝparas by going without and then you deponas what is left. Note also that geology borrows the word for sediment, so a river can deponi mud on a bank, and that is not a metaphor but the older Latin sense surfacing.