If you can't think of a good reason for your bad mood, it's unexplainable. And when you experience a strange coincidence, like running into a friend who you were just thinking about, that's also unexplainable. At the heart of this word is explain, from the Latin root explanare, "to make clear or make plain." Unexplainable adds the "not" prefix un- and the suffix -able, which expresses ability or capacity. Scramble those word parts together, and you've got a word meaning "not able to explain."