When a woman hits someone with her tits. It is roughly the female equivalent to a mushroom stamp. The root word 'milk' comes from the female mammary glands (i.e. breasts) ability to produce milk.
"She knocked my glasses off my face with her tits when she gave me that milkstamp"
"She has huge tits, so she gives great milkstamps"
"Will you tell your mom to stop milkstamping the neighbors with that giant rack of hers?
When a woman milkstamps someone by accident. That is, a milkstamp without the conscious intent of the woman giving the milkstamp.
If the milkstampee puts themselves into a situation where they are likely to receive a milkstamp and the milkstamper in question was not trying to give a milkstamp, it is still considered a spilled milkstamp. Even if the milkstamper enjoys the spilled milkstamp afterwords.
When a woman milkstamps someone and she is not wearing a bra or any brassiere like mammary restraint and she is not wearing a shirt or any torso covering clothing. I.e. a milkstamp from a completely topless, naked from the waist up, and bare chested, woman. It is consideredthe greatest of all milkstamps.