When you have your erected penis out and a group of people repeatedly slap it all at once until they either get tired or when it falls off, and by it i mean your peanits.
"Dude, I got a bunch of people to give me a Mega Slapper at the party last night!"
This is when someone or something is negative in a mega way. Immensely negative.
Jane: "Did you hear that I'm going to be a grandma? I'm so excited!"
Nancy: "Wow! I don't know how anyone would bring another child into this terrible world."
Jane: "Go piss off, you sour bitch! You exude nothing but mega-nega!"