arse from one's elbow, not know one's expr BrE vulg sl Why do you ask him of all people? He doesn't know his arse from his elbow Нашел кого спрашивать. Да он, бля, глуп как пробка,
NOT KNOW ONE'S ARSE FROM ONE'S ELBOW
Англо-русский перевод NOT KNOW ONE'S ARSE FROM ONE'S ELBOW
Глазунов. New English-Russian dictionary of modern informal English. Англо-Русский новый словарь современного неформального английского языка. 2012