in one ear and out the other

Inattentive; soon forgotten. This vivid image dates from Roman times. “The things he says flow right through the ears,” wrote Quintillian (Institutionis Oratoriae, c. A.D. 80). The sentiment was echoed by Chaucer and joined John Heywood’s 1546 proverb collection  (“Went in the tone eare, and out at the tother”). Thomas Hood punned on it in his “Ode to the Late Lord Mayor” (1825): “He comes in at one year, to go out by the other!”