It is a common nickname given to a beloved friend which literally means, In "Dutch" - a Sperm Rapist.
"Hey Merps.. MY little merpy werpy woo, you gone done poo poos in your pants again? Thats no good. You know I will always love you."
"Hey I am going to call you merps from now on because of that filthy act you did the other night!"
Anal sex with someone from behind whilst you place your hands around their throat and strangle them.
Jacob : Hey man, Im so going to 9th base with you tonight is that cool?
Dan Mcgree : Hell yes its cool, I want it badly I hope your ok with it though..