дырка от бублика

разг., ирон.

good-for-nothing; nonentity; empty shell; cf. a hole in a wall

Но тут фельдшер взноровился, откуда только у него и смелость взялась, и он шумит на Макара: "Хотя вас, - говорит, - и учёные люди лечили, однако сам вы, уважаемый, пробка в медицине!" Макар ему на это: "А ты в медицине - дырка от бублика!.." (М. Шолохов, Поднятая целина) — But this time the doctor really lost his temper - how he found the courage, I don't know! - and he bawls at Makar: 'You may have been treated by learned people,' he says, 'but you yourself, my dear fellow, know as much about medicine as a block of wood!' And to that Makar replies: 'And you know as much about medicine as a hole in a wall!'