late 14c., "accompanying, attendant" (especially as an auxiliary), also "descended from the same stock but in a different line" (distinguished from lineal), 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 "the side, flank of humans or animals, lateral surface," a word of uncertain origin.
Literal sense of "parallel, along the side of" attested in English from mid-15c. Related: Collaterally. Collateral damage is by 1873 in legal cases; in modern use, a euphemism for "the coincidental killing of civilians," an extended sense from c. 1968, American English, at first generally with reference to nuclear weapons.
"colleague, associate," 1510s, from collateral (adj.). Meaning "something of value given as security" is from 1832, American English, from phrase collateral security "property, etc., given to secure the performance of a contract" (1720), in which collateral (adj.) has the sense of "aiding or confirming in a secondary way."