Penis captivus is a possible urban legend describing an event that allegedly happens in rare instances during heterosexual intercourse when the muscles in the vagina clamp down on the penis much more firmly than usual, making it impossible for the penis to withdraw from the vagina regardless of erection status.
The first report of the penis captivus sexual phenomenon can be found in an 1884 article in the journal, The Philadelphia Medical News. The article was later discovered to be a hoax.
There is only one known report of penis captivus, in a letter to the British Medical Journal relating to an apparent case in 1947. According to the BMJ, this condition was otherwise unknown in the 20th century.
Penis captivus should not be confused with the relatively common condition of vaginismus.