What it means is that Arthur has nothing at all going for him. There's a silent word which as
a native speaker you should need no help with. The word is 'also'

Okay. I will provide your original sentence:
"I answered your dim but inarticulate assistant, Arthur."

As I am not familiar with how 'silent' words are used with 'but' to combine two synonyms, could you show me?

Certainly a better writer would have simply eliminated (silenced?) the erroneous and superfluous 'but':
"I answered your dim inarticulate assistant, Arthur."

You are eager to diminish Arthur's intellectual abilities and critique my writing ability. When questioning other individuals' ability to communicate, you should set aside that protective cloak of rustic jargon from your own writing, assuming you can actually write correctly.

if you truly are a charming dolt like your dear pDog, you can continue to write the way he does. But stop whining.