Misdoubt does not mean to doubt incorrectly. In a handful of older formations the mis- reinforced a verb that was already negative rather than reversing it, so misdoubt simply means to doubt or suspect — in use since the 1530s. The Countess in All’s Well That Ends Well gets the best line with it: “I could neither believe nor misdoubt.” Doubt itself goes back to the root that gave us two.