A running joke on /dbs/ where one posts a picture of Krillin kneeling on the ground with his jaw on the floor alongside text narrating Krillin reacting to various outlandish and bizarre situations, all of which invariably involve other men having sexual relations with Android 18. And then at the end Krillin yells "Yabba Dabba Doo!" to show his approval.

Has nothing to do with Fred Flintstone, whose catchphrase is "Yabba Dabba Doo!".