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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).
collateral col·lat·er·al (kə-lāt'ər-əl)
Indirect, subsidiary, or accessory to the main thing.
Having an ancestor in common but descended from a different line.
A branch of a nerve axon or blood vessel.
A collateral relative.