John wasn't stupid, at least, not as stupid as Sherlock may think, and he learned a few words of his own. The only problem was waiting for a time to use them.
(It helped that he learned ones relevant to things Sherlock would say.)
"You should read the writing I did on the different types of tobacco ash."
"Adoxography," John noted.
Sherlock glared at him. "Thank you for the compliment, but it is indeed of importance," he snapped.
John only smirked.