Several parts combine into one character.

苦 is a phonetic-semantic compound: 艹 (grass / plant radical) plus 古 (old / ancient). The composite originally referred to a specific category of medicinal herb — a long-aged plant whose taste had grown intensely bitter with time. The character's metaphorical leap is one of the most universal in human language: bitter taste → bitter experience → suffering. Across cultures and millennia, "bitter" carries the dual meaning of physical taste and emotional pain.

Korean reading "go." 苦痛 (gotong, pain), 苦惱 (gonoe, anguish), 苦樂 (gorak, "bitter and pleasant" — the ups and downs of life), 苦學 (gohak, studying through hardship — the classic Korean Confucian ideal of self-improvement through privation), 苦言 (goeon, harsh but well-meant advice), 辛苦 (singo, hardship). 苦學 deserves a note: the cultural ideal of a poor student who studies through every hardship is encoded in this single compound.

Mandarin kǔ, 3rd tone. 痛苦 (tòngkǔ, suffering), 辛苦 (xīnkǔ, "hard work / I appreciate your trouble" — used colloquially as a greeting to acknowledge someone's effort), 苦笑 (kǔxiào, wry / bitter smile), 吃苦 (chīkǔ, "eat bitterness" = endure hardship — a key cultural concept), 苦瓜 (kǔguā, bitter melon — the literal vegetable). The phrase 吃苦 captures something specifically Chinese: the willingness to endure pain for future benefit is treated as a cardinal virtue.

Japanese splits the dual meaning between two distinct kun-readings, one of the cleanest examples in the language. にがい (nigai, "bitter to the taste") — コーヒーが苦い (kōhī ga nigai, the coffee is bitter), 苦笑い (nigawarai, wry smile). くるしい (kurushii, "painful / distressing") — 息が苦しい (iki ga kurushii, breathing is painful), 苦しい立場 (kurushii tachiba, a painful situation). Same character, two readings, splitting the literal taste from the emotional pain. The on-reading ク (ku) covers compounds: 苦痛 (kutsū, pain), 苦労 (kurō, hardship — the standard everyday word for "trouble / struggle"), 苦手 (nigate, "bad at / disliked," literally "bitter hand").

Memory aid: aged (古) plants (艹) — bitter herbs, bitter experience.