impost comes from Old French impost, from Medieval Latin impostum, from Latin impositum ('that which is imposed'), neuter past participle of imponere ('to place upon'), from in- ('on') + ponere ('to put'). Morphologically, the prefixed shape matters: the initial element belongs to the recorded formation and helps explain the modern form's surface analysis. In present English, the noun sense is "a tax or duty, especially one levied on imported goods", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as impose, imposition, deposit, and positive. Its semantic neighborhood includes tax, duty, levy, and tariff, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Latin impositum and Latin ponere.