軽快
けいかい · keikai · JLPT N1 · na-adj
lively
🎈 軽 = light + 快 = pleasant: light and pleasant, so lively and casual
The 快 in 快適 means comfortable, and 軽 means light in weight. Something light and comfortable moves easily, which gives the lively, casual sense.
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.