苦 is a tidy pictophonetic: 苦 = 艹 + 古. The grass radical 艹 (cǎo) sits on top and carries the meaning, marking the word as a plant; 古 (gǔ) sits below and carries the sound, supplying the near-rhyme that gave kǔ its reading. The whole was originally the name of a specific bitter herb, a wild plant you would spit out the moment it touched your tongue.

The Shuōwén Jiězì glosses 苦 as 大苦, identifying it with a bitter medicinal plant. The seal form already shows the three-stalk grass crown above the 古 element, the same stacked arrangement that survives in the modern square. Nothing pictographic happens in the lower half: 古 is there for its voice, not its picture of an old mouth.

From the taste, the sense reached outward by an obvious bridge. A bitter herb is unpleasant to swallow; an unpleasant life is bitter to live. So 苦 stretched from flavor to hardship (苦难 kǔnàn, suffering), then to the effort that hardship demands (刻苦 kèkǔ, painstaking), and finally to the adverb that colors a verb with grim persistence: 苦练 (kǔliàn) is to train bitterly, to drill until it hurts.

Buddhism deepened the word permanently. When the First Noble Truth was rendered into Chinese, 苦 (kǔ) became the standard translation of Sanskrit duḥkha, the pervasive unsatisfactoriness of existence. To Chinese ears, the doctrine that 人生是苦 (rénshēng shì kǔ), life is suffering, lands with the directness of a flavor you already know.

Picture a bowl of dark herbal medicine cooling on the table, a clump of wild grass still floating in it. You raise it to your lips, and the taste is so bitter your face puckers. That clump of grass is the radical 艹 sitting on top of 苦; the bowl itself, old and well-used, is 古 (gǔ), the ancient sound underneath. Grass over old: a bitter old remedy.

Now follow the bitterness outward. The same drink that pinches your tongue is what 良药苦口 promises: good medicine tastes bad. Every compound on this page is some version of that swallow. 吃苦 (chīkǔ) is choosing to drink it; 刻苦 (kèkǔ) is drinking it daily until it sharpens you; 痛苦 (tòngkǔ) is when the bitterness reaches the heart, not just the tongue.

Hold the last note: 苦尽甘来 (kǔ jìn gān lái). You finish the bitter cup, and only then does the sweet arrive. Whenever you meet 苦, taste the grass-over-古 bitterness first, then remember it is the kind that, drunk to the bottom, turns sweet.