The story
Koselig is Norwegian, made from the verb kose with the ending -lig, the same ending as English -ly. Kose means to cuddle or to make a fuss of someone, and kose seg is to have a nice time. The great Norwegian dictionary NAOB finds koselig in a letter by Jørgen Moe from 1843. Moe, together with Peter Christen Asbjørnsen, collected the Norwegian folk tales.
A room can be koselig, and so can an hour spent while the coffee brews. A koselig fyr is simply a nice bloke. Norwegians even use it as a courtesy: koselig å hilse på deg means nice to meet you, and often only koselig is left of it.
English reaches for cosy, which suits the room. Koselig stretches further, to people and to time spent together, and it does not have the slightly small, cramped edge that cosy can have. Norwegian also has hyggelig, a relative of the Danish hygge that English took up in the 2010s, and that word is used more for pleasant people and meetings.
A scene: a Sunday in February, a cabin in the hills above Lillehammer. It is dark outside by four. Inside, wool socks dry by the stove and a pack of cards lies on the table. Nobody is in a hurry. Someone looks round the room and says only: 'Så koselig.'