The story
Gluggaveður is Icelandic, and the parts are plain: gluggi, window, and veður, weather. Put together they name a kind of day everyone in Iceland knows. The sky is a hard blue, the sun bounces off the snow, and from the kitchen it looks like a postcard. Then you open the door and the wind takes your breath away. Window-weather is weather for looking at.
English has a quiet connection to the word. Its own window is a Norse loan: Old Norse vindauga, literally a wind-eye, the hole in the wall where the wind came in. Icelandic went another way and kept gluggi for the opening itself, so its window-weather is the wind kept on the other side of the glass.
Britain knows the feeling well enough without having a name for it. A clear morning in January, frost on the cars, low sun across the fields, and an east wind that finds every gap in a coat. People say it looks lovely out, and then they stay in.
What the Icelandic word adds is the verdict. It does not say the weather is bad, and it does not say it is good. It says it is good from here. In a country where a bright day can still be bitterly cold, that is a useful distinction, and it fits in one word where English needs a whole sentence: lovely, as long as you are indoors.