The story
From Scots or Scottish English moch ("muggy weather") + -y. Moch is of uncertain origin but it is connected to dialectal English (Yorkshire) moke (“mist, fog; thick/close foggy weather”), which is related to if not derived from Old Norse mǫkkr (“dense cloud or fog”), dialectal Swedish moket ("cloudy, hazy"). Scots mochy can also mean "decaying, as due to dampness/heat", a sense which the DSL compares to Norwegian Nynorsk muggen (“moldy”), German muchen.
Etymology from Wiktionary, CC BY-SA 4.0.