a sort of joke about noun declensions in German
The word decline has two meanings in English. If you refuse an offer, you decline it. And (2) if you change an adjective, verb, adverb or noun according to its use in a sentence, you decline it. A friend once told me that he would rather decline a drink than a German noun.
I think that's a joke Mark Twain came up with in his famous piece regarding the German language.
"I heard a Californian student in Heidelberg say, in one of his calmest moods, that he would rather decline two drinks than one German adjective." from The Awful German Language ( https://www.cs.utah.edu/~gback/awfgrmlg.html )