Phonetic loan from English 'bye-bye,' written with 掰 (normally meaning 'to break apart by hand') purely for sound. Casual and cute, common in texts, voice messages and especially Taiwan usage. For formal contexts use 再见.
mistakes
Don't write it 拜拜 for the goodbye sense, that one is also used but originally means 'to worship.' Both spellings exist; 掰掰 is the more colloquial, internet-friendly one and tracks the English vowel better.
