苦口

lit. bitter taste (cf good medicine tastes bitter 良藥苦口|良药苦口) / fig. earnestly (of warning, advice)

mouth / classifier for things with mouths (people, domestic animals, cannons, wells etc) / classifier for bites or mouthfuls

bitter / hardship / pain / to suffer / to bring suffering to / painstakingly

earnest and well-meaning advice (idiom); to persuade patiently

good medicine tastes bitter (idiom); fig. frank criticism is hard to swallow