When an obscure observation or statement is made regarding something that has nothing to do with the original subject matter.
(While viewing a photograph of a drawing in a sketch book)
Randal: I drew this illustration in my sketchpad while there was no lights on! I used my cell phone as a light source!
Filmer: Fuck that!
Randal: Great, here comes the Filmerism.
filmer: I opened my sketch pad like this.
Randal: The fuck?