Borrowed fromPolishmożna with a change of the final morpheme, either to avoid an ending that would be anomalous for an adverb, or because of orthographic hypercorrection rooted in vowel reduction, or both. Synchronicallyмог-(mog-) +-ный(-nyj) +-о(-o) (but the form **мо́жный does not exist, unlike Polishmożny). Compare Ukrainianможна(možna).
мо́жно• (móžno) (impersonal, invariable)