苦しい
くるしい · kurushii · JLPT N3 · i-adj
tough
🏋️ 苦 = bitter, suffering; kurushii sounds like crush-y: a crushing effort
The kanji 苦 means bitter or hardship, and also appears in 苦い (nigai, bitter). Add the adjective ending しい and you get a feeling of hardship: kurushii, crushingly tough, like being out of breath on a steep climb.
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.