Nominalised form of the infitive 'to plug'. The gerund form broadens its contextual use and allows its employment in noun, gerund, and participle clauses.
Yo dawg, I was plugging for you hard last night with your boss. Expect a promotion soon.
The act of sticking your finger inside the hole on top of a penis, jacking it off, letting pressure build up, and pulling out, so the come shoots all over.