Muggy weather is unpleasantly warm, and the air itself feels wet — as if a thin, warm mist is sitting on your skin. You do not need rain. The wetness is already in the air, so you sweat, clothes cling, and a room with no wind feels heavy. English speakers use it for summer days, nights before a storm, and places near the sea or a river. It is a homely, everyday word, not a science word. Though actually, heat alone is not enough, and damp air alone is not enough. A hot dry wind is not muggy, and a cold wet morning is not muggy either. You need warmth plus that sticky wetness. People also stretch it a little: “It's so muggy in here” can mean a closed room feels thick, even indoors.