1 definition by BeforeChrist

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.
Hong Kong:
Cayden just said a lame joke to flirt girls, he didn't realize it was really cringy and on9.
Look! Ken just did his hair bald and it definitely doesn't suit him, it looks so on9 lol.
Matthew is very on9, he always slap his classmates' asses just to irritate them for no good reason at all! Then he escapes and laughs in a cheeky way, I genuinely don't understand his kind of humour.
US:
Memes are so on9 yet entertaining at the same time!
Filthy Frank is the most on9 youtuber I've ever seen.
UK:
Undoubtly, Mr. Bean is the on9 representative of cartoons.
by BeforeChrist March 12, 2019
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