盛装
せいそう · seisou · JLPT N1 · noun
wear rich clothes
盛 — 成 (chéng, phonetic) + 皿 (vessel)
A phono-semantic compound: 皿 (a dish or vessel) gives the meaning, while 成 supplies the sound. The original sense was grain heaped into a ritual vessel as an offering, hence 'to fill' or 'contain' (chéng). From 'full to the brim' it extended to 'abundant, flourishing, prosperous' (shèng).
装 — clothes 衣 + strong 壮 (phonetic)
装 is a semantic-phonetic compound. The bottom 衣 ('clothes') gives the meaning, since the original sense was packing or dressing—putting clothes or goods into a bundle. The top 壮 supplies the sound. Meanings extended from 'to dress/attire' to 'to load, install, pretend'.