a) a joke that is only funny to the one who is telling it.
b) jokes that cause akward silence, until someone says that it was a robby joke, causing laughter.
c) humorous statement that has bad timing causing everyone to look around at each other to see if anyone finds humor in what was just said.
Steve: dude, emily, your mom is so hot.
Kevin: she is hotter than hot pockets out of the microwave! hahahahahaha
Steve: ...dude, no robby jokes. those are gay.