Any idea, craze or cultural phenomenon made up from otherwise long dead and/or unlikely parts but has for some reason recently sparked general interest, becomes a monster, and then when your thoroughly sick of the bastard you cant get rid of it!
Fucking hell, that bastard annoying frog doing engine noises has become a right Frankensteins Monster! Pass the pitch fork, someone!
In reference to the fictional character in Mary Shelly's novel Frankenstein. What happens when you mention a thing you like (e.g. band, movie, etc...) to someone and they become a cult follower of said thing, taking it to the nth degree.
Clarence: I watched Fight Club with Nick yesterday and I think I've created a Frankenstein's Monster; now all he wants to do is fistfight people and pretend he's got split personality disorder.
Joe: Dammit, is that why he hit me in the ear just now!?