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The term 'gweilo' is a Cantonese term that is used to denote Caucasians. It actually translates roughly to "Spirit/Ghostly Fellow" - referring to the colour of the skin. It is said that in their first encounter with the Europeans, the southern Chinese thought that the they were spirits - (which is interesting as many indigenous Australians stories say the same thing about their first encounters with the Europeans). So, a woman might be referred as "gwei poh", a boy "gwei zai" and a girl "gwei mui". While it does sound racists - many expatriates in Hong Kong actually own the term and would even refer to themselves as such. It's not necessarily a derogatory term, though it can be used in a derogatory way, depending on the tone. If you are really interested - there's an autobiographical book titled "Gweilo: Memoirs of a Hong Kong Childhood" by Martin Booth.
Incidentally - I didn't grow up using those terms to describe Europeans - in Malaysia and Singapore, ther term used was 'ang moh', which translates to "Red/Copper Hair", again describing that which is the most obvious difference between black haired Asians and other foreigners. 'Ang Moh' on its own is purely an adjective and not meant to be derogatory but again, in the most racist instances (yes! they exist everywhere) the adjective could be added to any other noun that are much less flattering. The terms 'ang moh' is Hokkien, not Cantonese, and it always amuses me that it is actually similar to Aussies use of the term 'rangas', except, 'ang moh' refers to ALL Caucasians, not just our beloved copper fiery ginger friends.