collateral

Recorded in English since around 1378, from Old French, from Medieval Latin collaterālis, from Latin col- ("together with") plus the stem of latus ("side").

The facts available don't trace a specific shift in meaning over time, only the layered borrowing from Latin through Medieval Latin and Old French into English.

It shares its second root, the Latin stem of latus ("side"), with the plain adjective "lateral" — collateral is essentially "lateral" with "together" bolted on the front.