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.