NOT a typo.

馬の耳に風
(Muma no mimi ni kaze;
“Wind in a horse’s ear”)

Definition:

Ignoring something somebody has said, especially people’s opinions or criticism that you don’t want to bother with. “In one ear and out the other.”

Breakdown:

We begin this noun phrase with the noun 馬, “horse,” which you’d expect to be pronounced uma but here is pronounced muma for some reason. This associative particle の (no) marks it as the possessor of the noun 耳 (mimi), “ear.” This in turn is marked by the directional particle に (ni) as the target to which is applied the noun 風 (kaze), “wind.”

Notes:

This is the む (mu) entry in the Kyoto iroha karuta set. No, none of my normal sources explain why they went with mu for this one. Is it dialectical? Court lady word-play? (That’s a real part of some etymologies!) There’s a claim that this brief saying is derived from the four-character compound 馬耳東風, but while Japanese does have some m/b overlap, the ba reading there doesn’t really seem like a great fit for muma.

Compare and contrast this phrase with both the above yojijukugo and with close synonym 馬の耳に念仏 (uma no mimi ni nenbutus), “a Buddhist prayer in a horse’s ear.” Beyond this there’s apparently an entire picture-book’s worth of similar sayings… including, but not limited to:

• Dogs and the Analects of Confucius
• Cats and Buddhist sutras
• Pigs and Buddhist prayers
• Rabbits and a style of Shinto prayer called 祭文 (saimon)
• Cattle and koto music

Example sentence:

「もう、あいつ、馬の耳に風だから何言っても無駄だった」　「ムマの耳？」　「ウマの耳と同じ意味だよ」　「ムマの耳はウマの耳に似ている？」　「あんたもわざわざウマの耳にならない方が良いんだよ」

(“Mou, aitsu, muma no mimi ni kaze dakara nani itte mo muda datta.” “Muma no mimi?” “Uma no mimi to onaji imi da yo.” “Muma no mimi wa uma no mimi ni niteiru?” “Anta mo wazawaza uma no mimi ni naranai hou ga ii nda yo.”)

[“Ugh, it’s like talking to a brick wall; there’s no point talking to them no matter what I say.” “You talked to a wall?” “No, it means that that jerk didn’t listen to me.” “A wall of jerky?” “You don’t need to turn into a wall either.”]