Let's get straight down to business, especially since there's no polite way to translate to this.
Cojidon is related to coger, and coger means to catch, or to take, depending on the context. But in Mexico and some other parts of Latin American, it also means to tener sexo. But a more accurate translation is "to fuck".
While that's nice to know, it still doesn't explain what cojidon means. By now I'm sure you've guessed it has something to do with sex, since it's related to coger. So what does it mean?
A cojidon might be loosely translated as a "mighty fuck". But we'd say something like "I'm going to fuck the shit out of you".
So if you've performed as promised, your parter (pareja) might say something like:
Me dio un cojidon -- He (or she) fucked the shit out of me.
Heck, you might even hear that you're muy chingón (you're the shit, or the best) in the bedroom.
And if you going to go around handing out cojidons, be sure to practice sexo seguro (safe sex).
¡Hasta la próxima!