The act of a man having sex with a woman by repeatedly stroking his penis under her armpit. The term is similar to armpit sex, the armpit fuck, or dick in a headlock. However, it differentiates itself through the use of household items such as butter, margarine or cream as a lubricant to entice the woman to lick the penis as it protrudes from her armpit.
Man, I got a mean burn in my armpit from a Molly McButter last night.
I had to de-grease my penis after giving some Molly McButter to my old lady.