苦恼
kǔnǎo · HSK 7
vexed
苦 = bitter + 恼 = annoyed; a bitter annoyance is being vexed and worried
苦 (kǔ) means bitter and 恼 (nǎo) means annoyed or irritated. Stack them and you get a bitter irritation that gnaws at you — the worried, vexed feeling of 苦恼.
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.