(Chuugen mimi ni sakarau; “Good advice hurts your ears”)
When someone is truly trying to help you, they acknowledge your faults and failings and give you good advice based on that honest assessment. But it can be hard to listen to this sort of critical assessment.
忠言 (chuugen) is advice – generally good advice; the characters literally mean “loyal word.” Note that there is no particle here to mark the noun 忠言‘s function within the sentence; in an aphorism, that level of grammatical detail simply isn’t necessary. 耳 (mimi) is ear(s), and 逆らう (sakarau) is a verb meaning “to defy,” “to go against.”
The particle here is where things are a little weird, from a native English speaker’s point of view. I would have expected the ears to rebel against the advice. But no, the particle に (ni), marking direction or location, is attached to 耳, meaning that the advice “goes against” the ears. It’s not as deeply counterintuitive as some of the gymnastics you’ll see the Japanese language perform, but it does at least stand as a warning not to make assumptions.
Using 忠告 (chuukoku), which can also mean “advice” or “warning,” is incorrect.
This kotowaza also exists in yojijukugo form as 忠言逆耳 (chuu.gen.gyaku.ji), and as a much longer phrase that compares the good advice to bitter medicine. Its origins are supposedly in China, from the Family Sayings of Confucius – an apocryphal text that may or may not have anything to do with the philosopher or his disciples.
(“Chuugen mimi ni sakarau kamo shirenai kedo, konshuu wa asobazu ni benkyou ni shuuchuu shita hou ga ii nda yo.”)
[“You probably don’t want to hear this, but it would be better if you focused on your studies this week, and skipped gaming.”]