刻苦 (kè kǔ) is an adjective meaning 'diligent' or 'hardworking', describing someone who studies or works with great effort, perseverance, and self-discipline. The character 刻 (kè) originally means 'to engrave' or 'to carve', suggesting deep focus and intensity; 苦 (kǔ) means 'bitter' or 'arduous', evoking the idea of enduring hardship willingly. Together, they convey a sustained, almost rigorous commitment to improvement — not just working long hours, but doing so with purpose and resilience.

This word is commonly used in academic, professional, or skill-development contexts — for example, praising students who practice tirelessly, athletes training through fatigue, or artists refining their craft over years. It carries a positive, respectful connotation and often appears in formal praise, motivational speeches, or parental encouragement. Unlike more neutral terms like 努力 (nǔlì, 'to strive'), 刻苦 emphasizes both mental tenacity and willingness to bear difficulty.