Sorry to be the one to break this to you but people do actually use this word. There are several molybdenum mines still running in the U.S. I believe. My father worked in one many many moons ago, but as always that's another story.

As to the question of how to pronounce it you could choose the wrong but semi-witty one of "molly be damned" which is sort of a running joke in the movie The Apple dumpling Gang Rides Again. I can't remember at this moment exactly how they pose the joke but the basic idea is no one can pronounce it so everyone chooses the molly be damned version.

The other choice I can think of is the real and actually fairly simple pronunciation once you know it.

mu lib de num

all the vowels soft...