Pubic wig, often used by prostitutes who have been infected with or are trying to avoid pubic lice.
Amy: "Karen, my crotch is itchen pretty bad, there may be lice... what should i do?"
Karen: "Shave it all off and get yourself a merkin!!!"
Person 1: "i met this prostitute and she was wearing a fucia pubic wig.. whats that called again?"
Person 2: " oh, person 1, thats just a merkin."