adjective/suʁ.dɛ̃ɡ/
completely deaf
“Dis donc, tata Marceline, dit Zazie, tu te fous de moi ou bien t’es vraiment sourdingue ?” — "Hey, Auntie Marceline," said Zazie, "are you taking the piss, or are you completely deaf?"
From sourd (“deaf”) + -ingue.