What's "Mao Li" (Cantonese) in mandarin? I've read an article which says that "Mao Li" in cantonese means "fool" in English.

What is the mandarin equivalent of "Mao Li"?

MandarinTools does not seem to return any useful results for Cantonese look up of "Mao Li".
It is one of the obscene language in Hong Kong.
It can be write as 茂利/茂李 (mau6 lei2 in cantonese, Màolì in mandarin), sounds like an English word "mullion"
In Cantonese, we use the word to describe a man standing still like a pillar, later use to describe stupid person.

Reference from https://hk.knowledge.yahoo.com/question/question?qid=7008062100602, I did a simple translation on the important parts


  是由20世紀初，英國人在香港教中國人建築而來。
  當時英式建築有好多大型窗戶和陽台，例如港督府，
  這些窗戶和陽台的主要支柱，英文叫mullion。
  當時的mullion全部都是使用很粗大的木柱，但只有立著的才叫mullion，
  橫的叫transom。當時香港人又不懂英文，聽見那些洋人指著一大根木桿猛叫[mullion] 漸漸就學著叫[茂利]，意思是一根根木棍或木柱，
  就像傻愣愣站著的人，後來變成用來形容一些愚鈍之人。