A pubic hair piece. A toupee for the pubic area/genitals. In the 1700’s when mercury was used to treat sexually transmitted diseases (Gonorrhea or Syphilis) one of the side effects was the loss of pubic hair. To disguise this condition, that was not cured by mercury, a Merkin was employed.
17th Century setting: Due to his treatment of the "French Pox" (syphilis) with mercury a merkin Gwendolyn used a Merkin to used to hide the side effects of hair loss and not alarm her husband.