I'd be curious to know if you would actually be sued. I mean, if you write a story and use the workd light sabre, and its really good, and you're really lucky, and it gets picked up, your publisher is likely going to suggest you change the name. Let's say that you refuse, but somehow the book gets published anyway and you sell a few thousand copies.
I think THEN you might find yourself in hot water. But even if that did happen, I suspect that you would have to make a significant amount of money with the book before anyone from Lucasfilm would bat an eyelash.
My advice:
If you're writing for fun and it's what you envision, go ahead and use the word lightsabre. No one's going to sue you for writing fan fiction. If you're trying to produce something marketable, you'll need to come up with a unique spin on it before a publisher will look at it anyway.