Maybe he does know what risible means and is in fact extremely well informed, much better informed than you are, to the point where offering sarcasm on the appa

Maybe he does know what risible means and is in fact extremely well informed, much better informed than you are, to the point where offering sarcasm on the apparent basis of absolutely nothing but what you've learnt from the internet is actually not a valuable contribution to the conversation but instead embarrassing. Have you considered this possibility as well?