The story
The adjectival sense's relation to adultery is the common theme that corruption or pollution has occurred; more at adulterate. By surface analysis, adulterate + -ed.
Etymology from Wiktionary, CC BY-SA 4.0.
adulterated
Today it means: Intentionally tainted with impurities (usually as motivated by cheapening the production of a food product or enhancing the psychotropic effects of a street drug). Full definition →