The word 居然 is formed by combining the character 居, which in its classical usage can carry a sense of "unexpectedly" or "how could it be", with the character 然, a classical suffix that turns a preceding element into an adverb meaning "in such a manner". Thus, the compound literally means "in an 'unexpected-how-could-it-be' manner", crystallizing into a single adverb expressing surprise at an unforeseen outcome.