回信
huí xìn · HSK 5
to reply
💌 回 = return + 信 = letter; a letter sent back in reply
回 means to return or go back, and 信 means letter or message. Together 回信 is the letter you send back — your reply. The classifier 封 (fēng) counts sealed envelopes.
回 — enclosure 囗 + smaller 囗 inside
回 is a pictograph of a swirling whirlpool or coiled spiral, shown as nested squares in modern form. The original meaning was 'to revolve' or 'turn around,' which extended to 'return' and 'a time/occurrence.' It also appears as a phonetic in characters like 茴 and 廻.
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.