Hop off my johnson means in a different phrase "get off my case". Used mostly when youre just joking around with your friends but can also be used when trying to get someone thats attempting (and probably failing harshly) to tell you off. They get confused, you walk out feeling good that you got away from that.
Person 1: "I bet you've never even been kissed before, have you?"
Person 2: "Hop off my johnson!"
*person 2 walks out while person 1 is still in a state of confusion.