"You are a quiet man."
Translation:Je bent een rustige man.
The 'man' in the sentence is referring to gender, just like it is in English. 'Meneer' is not a formal way of specifying someone's gender, just like mister/sir is not in English.
Hence you can say: U bent een rustige man. But you don't say: U bent een rustige meneer. Just like in English you don't say: You are a quiet mister/sir.
Please explain further. 'Cause if I 'member correctly, Duo recognizes "meneer" as "gentleman". And while today we don't say "You are a quiet mister/sir", it has been used such at times in the past. Probably colloquially.
At least one of my Grandma's antecedents spoke to her own husband like that; she also, never in her life, ever called him by his vorname. She always addressed him as "Mr.....", and he likewise, or so we were told.