nipple knife
A nipple knife is when a females nipple gets really hard and could be used to cut open a potato or any other hard object a knife can handle.
Last night my girl friend had her nipple knife out cutting the potatoes and slinging them into the hot boiling water to turn them into mashed potatoes.
Knife nipples
When a male/female breast are super perky
Andrea has the sharpest knife nipples I have ever seen.