快乐
kuài lè · HSK 2
happy
😊 快 (fast) + 乐 (joy) = quick joy → happy
快 means fast/quick and 乐 means joy/music. Together 快乐 literally reads as quick-joy, the compact compound for happy. The character 乐 also appears in 音乐 (music) — joy and music share the same root.
快 — heart radical 忄 + 夬 (phonetic)
快 is a semantic-phonetic compound. The heart radical 忄 carries the meaning, since the original sense referred to a feeling of pleasure or delight. The right side 夬 supplies the sound. The meaning later extended from 'pleased' to 'quick, fast', the dominant modern sense.
乐 — stylized depiction of a wooden instrument with strings
Originally a pictograph of a wooden frame holding strings or bells, representing a musical instrument. The character first meant 'music' (read yuè), and by extension came to mean 'joy, happiness' (read lè), since music brings pleasure. The simplified form 乐 streamlines the older 樂.