信封
xìn fēng · HSK 3
envelope
✉️ 信 (letter) + 封 (seal/cover) = sealed letter cover
信 means letter or message; 封 means to seal or a sealed envelope. Together 信封 is literally the cover that seals a letter — an envelope. 封 also appears in 封面 (cover) and 封口 (seal an opening).
信 — person 亻 + speech 言
信 combines 亻(person) with 言(speech), depicting a person standing by their words. The original sense is 'truthfulness' or 'sincerity'—what a person says can be trusted. From this it extended to 'believe,' 'trust,' and later 'message' or 'letter,' since a letter carries one's word.
封 — earth 土 doubled (圭) + inch 寸 (hand)
Originally a pictograph of a hand planting a sapling on a mound of earth, marking a boundary. From this came the meaning of granting fiefs to nobles, then sealing or closing off territory. The modern senses 'to seal' and 'envelope' both stem from this idea of enclosing.