The act of making an erect penis flaccid or even go inwards, mostly by showing shocking, gross and/or gory pictures or vidoes. This is called the psychic de-erection. A psychic de-erection can actually be caused by basically anything as long as the person looses his erection from the picture, sound, thought, or non-physical influence.
De-erecting a penis can also be performed physically by smacking the gland hard with the palm of ones hand. A myth shown in movies says that this act is usually done by smoking hot nurses. Though, it is usually done by either a man or a fatty nurse named Ursula.
Another myth says that a physical de-erection is very painful, and that is indeed true.
Oh my god, last night Brad showed me this video called "2 guys 1 horse." My penis totally de-erected!
That fat chick last night totally gave me a de-erection!
"Did you hear that Simon went to the doctor last night?", Said Rick. Jane looked shocked. "How come?", she said. "He just got a de-erection, no big deal"