Mid 18th century; earliest use found in Alexander Ross (1699–1784), poet. Probably the reflex of a borrowing from early Scandinavian (compare Norwegian (Nynorsk) mo close, sultry, and also Old Icelandic móða condensed vapour, mist on a warm day, haziness, Norwegian (Nynorsk) moe heat haze, and the second element in Norwegian kornmo, Danish kornmoe, both in sense ‘summer lightning’), further etymology uncertain; perhaps related to the Scandinavian base of Old Icelandic mór.