collateral

Where it comes from

Collateral came through Anglo-French from Medieval Latin collateralis, literally side by side. It combined col-, the form of com- before l meaning with or together, and lateralis, of the side, from Latin latus, side.

How the meaning developed

Things beside the main line became secondary or supporting. A cousin is collateral kin because the relationship branches beside direct descent; collateral security supports a primary debt from the side; collateral damage is harm outside the intended target.