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stunt cock

n. A phallus used to sub in for a pornographic actor's actual penis, either another man's or a realistic facsimile

Wikipedia
Stunt cock

A stunt cock is the term for a substitute (sometimes prosthetic) penis that is used during the shooting of pornographic films.

The stunt cock is used in an extreme close up so as not to identify its bearer, the goal being to deceive the viewer into thinking that the stunt cock is actually the penis of the main actor. Thus, a stunt cock is analogous to a " stunt man," who anonymously does dangerous live-action sequences in place of the main actor. However, a stunt cock seldom performs dangerous acts on film. Though the term draws its name from this term, its practical application is more analogous to a body double.