苦い
にがい · nigai · JLPT N4 · i-adj
bitter
☕🍫 nigai sounds like knee-guy: the knee guy winces at bitter black coffee
苦 is built from the grass radical 艹 on top of 古 (old): old herbs taste bitter. The same kanji also reads kurushii (painful), so bitterness and hardship share one character. い marks it as an i-adjective.
苦 — grass radical 艹 + 古 (old)
Originally referred to a bitter-tasting plant, with the grass radical 艹 indicating its botanical nature and 古 providing the phonetic element. The meaning extended from the literal bitter taste to figurative senses of hardship, suffering, and painful effort.