There are kanji that give much clearer meaning to phrases. Like 'fu' - 不 - a negative prefix that's used like 'un-', 'not' or 'non-'.
If you see a bunch of kanji with 'fu' at the start, you know it's probably not going to be good. Work out what the rest of it means, and then use the opposite:
不満 - 'fuman' - unsatisfied
不燃ゴミ - 'funen gomi' - non-burnable garbage
不足 - 'fusoku' - not enough (insufficient)
不自然 - 'fushizen' - unnatural
Then, as with nearly everything about learning Japanese, we get to the exception.
不知火 - 'shiranui' - mysterious lights on the sea
First of all: I LOVE that Japanese has a word for this.
I didn't know about 'shiranui' until I started looking into phrases starting with 'fu'. It doesn't, clearly, which explains a lot.
Also, I'm not in the habit of using this expression on a day-to-day basis. I've never lived by the sea. A vicious seagull once stole my battered cod outside a seafront chip shop, and now being near beaches is traumatic.
Moving on... Those first two kanji - 不知 - are more commonly read as 'fuchi', meaning 'ignorance'. See, that's got a 'fu' in it.
So we have an 'ignorant fire', really. I guess any fire that tries to live out on the sea would be ignoring the obvious pitfalls of that plan.
The 'shiranu' reading comes from Old Japanese. As does the legend of 'shiranui' as mysterious will-o-the-wisp things that only appear at certain times.
My plan was to bring this post full circle by saying that the origin/cause of 'shiranui' is still unconfirmed. Y'know, 'fukakunin'... except that's not how you say that, either. (It's 未確認 - 'mikakunin', if you're interested).
I guess that's just me being unlucky - 不運 - again. Damn that seagull.