A lie; something that is utterly made up and/or false.
Rebecca: Jordan is a gullywhopperor!
Erin: What gullywhopper did he tell you this time?
Rebecca: He tried to tell me that Kyle's trout is 12 inches long.
Erin: He should really stop gullywhoppering to you...it's just not funny anymore. I mean we all know that it's only 11 inches.