late 14c., "accompanying," also "descended from the same stock," from Old French collateral (13c.), from Medieval Latin collateralis "accompanying," literally "side by side," from Latin com- "together" (see com-) + lateralis "of the side," from latus "a side" (see oblate, n.). Literal sense of "parallel, along the side of" attested in English from mid-15c. Related: Collaterally.

16c., "colleague, associate," from collateral, adj.. Meaning "thing given as security" is from 1832, American English, from phrase collateral security (1720).