1734, "to render neutral" (in a chemical sense), from French neutraliser (17c.), from neutral (see neutral (adj.)). Meaning "to counterbalance, render inoperative, invalidate" is by 1795; as an underworld euphemism for "to kill (someone)," by 1987. The intransitive sense "remain neutral" is older (1660s) but is now obsolete. Related: Neutralized; neutralizing.