不但
búdàn · HSK 2
not only
不 = not + 但 = only/just; together 'not only'
不 negates and 但 means 'only/just/merely', so 不但 literally reads 'not merely' — i.e. 'not only (this, but also that)'. It sets up a follow-up clause, usually paired with 而且 or 也.
不 — single pictograph; bird flying upward
Originally a pictograph in oracle bone script, traditionally interpreted as a bird flying up toward the sky and unable to come down, suggesting negation or refusal. Over time it became the standard negation word in Chinese, used before verbs and adjectives to mean 'not'.
但 — person radical 亻 + dawn 旦
A semantic-phonetic compound: 亻 (person) gives the meaning side and 旦 (dawn) supplies the sound. Originally it depicted a person baring the shoulder, an old sense preserved in 袒. Over time 但 was borrowed to write the function word meaning 'only' and later 'but'.