Vulgar
Used in Hong Kong
Pronunciation: "on-gau" ("on" in English, "9" in Cantonese)
Meaning: stupid, thinking stupid
More: on99
Chief Exceutive Mr.Tung: If China is good, Hong Kong will be good. When Hong Kong is good, we are good too.
Citizen A: He's on9.
Citizen B: He's on99.
by HKboy February 21, 2005
adj. Very stupid or foolish.
Pronounced "on gau" (The 9 is pronounced in Cantonese)

Cantonese slang used mainly by young Hong Kong Chinese. First originated on the Internet (isn't everything) but soon spread to everyday speech.

Originates from two words:
The first part "on" comes from "On ghuy" - a cantonese slang word meaning foolish.
The second part "9" comes from "gau" - a cantonese slang word meaning penis.

Also used as part of the phrase "on9 zai" meaning foolish man/boy.
Dude 1: Wow, that silly mainlander squatting on the pavement looks so damn on9.

Dude 2: Shut up, you on9 zai.
by The loch monster August 9, 2006
on9 (憨鳩) is a Cantonese swear word which means foolish, stupid and brainless.
It is pronounced as 'on' + 'gau'. ('gau' is the Cantonese pronunciation of '9')
It is a friendly swear word to laugh, scold or discriminate other ppl.

on99 (憨鳩鳩): very foolish
on9 zai (憨鳩仔): a foolish guy
on lun 7 9 (憨撚柒鳩): extremely stupid, mostly used to scold others when angry
on gui (憨居): a less violent way to say on9
Person 1: The guy steps on a piece of shit and slips.
Person 2: HAHA, he is really on9.
by ccthz November 22, 2021
It is short term of online

Or
Feels very happy, short term as on cloud nine Or
Word for appreciation, which is upper than 1-9, and greater than them, for appreciating people who done very good advancement
When he on9s, he feel on9, and do a very on9 thing
by PolyMoster April 22, 2022
A slang from Hong Kong which means extremely stupid and dumb
e.g. Ricegum is so on9 that everybody make fun of him
by Nejshhd July 8, 2021