MerkinA merkin is a simple substitute vagina made of plastic, rubber, or a similar material, and designed for use in masturbation or as an adjunct to sexual intercourse. They are made in varying degrees of anatomical faithfulness, in some cases including copies of the clitoris, labia, and pubic hair.
Most men don't find them particularly stimulating.
More serious efforts at building sex machines that are stimulating to men have been made, with considerably greater attention to texture, response to pressure, motion, and temperature. Some have been successful. Most are one-of-a-kind quasi-artistic efforts.
See also dildo
Merkin is also a slang term for an American.