Technically a compound of Mist(“dung”) +Wetter(“weather”). Mist! is a rather common interjection equivalent to English “crap!”. However, the word is not freely prefixed to nouns, which makes it seem likely that Mistwetter is originally “misty weather”, pertaining to Middle Low Germanmist(“mist”). The association with Mist (“dung”) would thus be secondary. Compare, on the other hand, compounds like Mistkerl(“nasty guy”), Miststück(“mean person”).