What's "Mao Li" (Cantonese) in mandarin?
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。