A fork of Rural Dictionary
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.