Meaning: Unintended civilian casualties or damage in a war.
Origin: ‘Collateral damage’ is a euphemism adopted by the US military in the mid 20th century. The word collateral derives from the Latin word ‘collateralis’, which means ‘together with’. So, when an intended target is struck and damage or death is caused nearby, that damage is ‘collateral’. Only the US military, with their reputation of obfuscation, could translate ‘innocent people were killed’ into ‘damage was caused nearby’.
In May 1961, T. C. Schelling wrote an article titled Dispersal, Deterence, and Damage, which referred to the term:
The USAF Intelligence Targeting Guide defines the term [that is, ‘collateral damage’] “unintentional damage or incidental damage affecting facilities, equipment, or personnel, occurring as a result of military actions directed against targeted enemy forces or facilities. Such damage can occur to friendly, neutral, and even enemy forces”.
The expression ‘collateral damage’ wasn’t invented by the military, they took on a term that had existed throughout the 20th century. Here’s an example from the New-York Tribune, February 1905, referring to plans to build watercourses:
I do not believe the State should build reservoirs, aqueducts and similar works. I believe they should merely receive plans… to protect the industries along the streams and also on the collateral damage and other questions involved.
The expression ‘collateral damage’ is now used more widely and isn’t restricted to being a military euphemism. Any unintended consequence, major or minor, might be so described. In any ‘you can’t make an omelette without breaking eggs’ scenario the breaking of the metaphorical eggs might be described as ‘collateral damage’.