From the Marmite FAQ: (http://www.gty.org/~phil/marmite.htm - I think this has been posted here in the distant past, but I'm too lazy to LIU and give credit where credit is due...)

A marmite is a French stock pot or cooking pot—like the one pictured on the front of the jar and shaped somewhat like the jar itself. The name of the French pot is pronounced "mar-MEET." The product name may have been derived from a famous French soup, petite marmite.

Good thing I looked - I was about to guess mar = sea, since the stuff tastes like it has the salt of a thousand oceans in every jar.