When a person dilutes your alcoholic drink with the soul intention of keeping you more sober than it was your intention to be.
You are at a party (or any drinking situation). You are getting drunk. You are getting too drunk. A person mixes your drink for you with less alcohol and more mixer than you intended to consume.
"She's getting too drunk. I'm going to Mormon her drink so she doesn't puke on my bed later."
She drinks the drink.
"Hey! You Mormoned my drink!"