to cause an object or person to become unsavoury, unattractive, boring, or otherwise less valuable simply by association. For example, if you like the Ramones a lot but are stuck in a car for eight hours with an obnoxious loser who enjoys calling out Ramones lyrics a full second before Joey actually sings them, the Ramones can quite possibly become tainted.
I don't know the exact origins of this term, but I never heard it used very often among my friends until I wrote a musings entry unofficially entitled "Theresa taints trip."