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
New English-Russian dictionary of modern informal English. Новый Англо-Русский словарь современного неформального английского. 2012