a ten dollar Hong Kong banknote (a 10HKD note used to be green in the past); a mud crab
Definitions (粵典–words.hk)
名詞、俚語、口語、香港
香港舊十蚊紙幣嘅俗稱,已於1994年停止發行,因為用綠色作為主色,所以被稱為「青蟹」 old $10 Hong Kong note, stopped issuing in 1994; as the main color is green, it is called "Green Crab"