A mispronounciation of the Led Zeppelin song D'yer Mak'er. When asked about the title and the song, Robert Plant stated that it is pronounced "jermaker". It is a play on the word Jamaica. If one listens to the music of the song, it has a very heavy reggae tone, beat and even the guitar notes are all reggae (with Zepplin influence). It was the only song that the boys did with reggae influence and they decided to name it D'yer Mak'er in reference to the well know joke.
from Joke About Jamaica