When you take a potato and peel it into a phallus. The potato can then be used as a sex toy. Different variations include a horse-shoe shape that can be used for double penetration. After use the potato can boiled and eaten.
Person 1: I heard that ranga plays with his food if you know what I mean...
Person 2: Yea I heard they like to do the potato pump.
A potatis is a swedish word, meaning that you are completely useless. Originally the word translated to potatoe in English, but that meaning has been lost.
You are such a potatis, failing to throw a granade properly without killing your friends.