collateral (adj.)

late 14c., "accompanying," also "descended from the same stock," from Old French collateral (13c.), from Medieval Latin collateralis "accompanying," literally "side by side," from assimilated form of Latin com "with, 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.

collateral (n.)

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