What else, but a limp dick. A penis that is flaccid, or unable to maintain an erection upon arousal.
Can refer to a person who is impotent.
Also relates to the state of the penis, after some form of sexual satisfaction, either from copulation, or masturbation.
Sometimes used in bragging rights, in claiming they have a dead stick from so much sex.
Also used on occasion to imply one's inability to perform sexually, at that moment in time.