Verb. The act of ejaculating in a drinking glass, mixing it with cola (Root Beer and Dr. Pepper are acceptable substitutes), and then drinking it*. This word is usually used as a derogatory term directed at someone else.
*Note: Having somebody else drink it is not considered 'slemming' although it is quite funny.
Ryan: So what are you going to do over vacation?
Tyler: I don't really have anything planned out yet.
Ryan: You probably will be slemming the whole time.
Tyler: No way, Jose! That is sick
Ryan: But that is what you're going to do, right?
Tyler: (Sarcastically) Well...
Ryan and Tyler: HAHAHAHA