Taint means to contaminate. If you don’t want to taint your drinking water, don’t use an old gas can as a water pitcher. Tainting something makes it impure.
To taint something is to spoil or corrupt it, whether it’s water, food, or even a person’s soul. These days, taint gets in the news if a company accidentally taints meat with salmonella bacteria, for example. If you add something poisonous to a substance, you taint it. A silly person might say when you taint something, t’aint right.