I'm German and I've never heard of this word but will use it from now on. The fun thing about German is that you can smash any number of nouns together so you could make it Sonntagmorgensleere or Wintersonntagsleere etc.

It's only an orthographic convention. English and all other languages can make compound nouns of arbitrary length, and the parts can but don't have to be nouns. In fact, there don't have to be any nouns in a compound noun! E.g. backup.

English just puts spaces between the parts usually, but as I understand it, this is unusual among Germanic languages.