The phrase "plug it" refers to when one chooses to start rolling (consume ecstasy) by putting the pill (also known as a "bean") into the anal cavity to "roll harder". Contrary to belief, this method actually helps one roll harder and longer.
"Alright man, I scored four beans. What are you going to do with your two?"
"Dude, don't laugh but I'm totally gonna plug it."
"Oh man, bro. Have fun and wash your hands when you get back, okay?"