The idiom "莫名其妙" originates from a historical anecdote about a man named Mr. Mo (莫) who witnessed a strange and wondrous fire (妙火) whose cause and nature he couldn't explain. Over time, the literal meaning of "莫" (a surname) evolved into its more common function meaning "cannot", and "妙" (from the specific "妙火") broadened to mean "wonder" or "mystery". Thus, the phrase conceptually formed to describe the state of being so baffled by something strange and wonderful that one "cannot name its wonder", perfectly capturing the feeling of being utterly puzzled by the inexplicable.