Magikarping or to Magikarp a person, is for one to continue sex until the moment in which you are to ejactulate, at which moment you must pull and and flop your penis in the pool of sperm you have just released upon the partner, whilst shouting 'Magikarp-karp-karp' and like Magikarp noises
It can be seen as a legendary act if carried out and a humiliation to the victim
Have sex until
*about to cum*
Cum on Belly
Flop Penis into the pool of Cum
Shout like a Magikarp
The start of this seems to be from anime conventions and a spin-off and response to planking, as the action is usually recorded not photographed.