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Mostly used in Hong Kong Cantonese slang word Spelling in english: On gau/gao 9 is pronounced "gau" in Cantonese Which also means penis or dick It means foolish, stupid, dumb, especially when describing someone's actions or speech even appearance but not specifically about the target's intelligence. This word is commonly used among youngsters, as it is mildly offensive yet usually in a funny way. Still it can be aggressive and provoking sometimes if the tone is not friendly or used in a swearing sentence. In numerous cases, it is transformed into on99 or on-lun-9. The meanings are basically the same.
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