Gweilo, also pronounced as gwailou is a Cantonese slang word for ‘western foreigner’. Gweilo is being used to typify ‘white’ people or to be more precise: people who are not Chinese.
Gwái means “ghost” or “devil”, and lóu means “man” or “guy”. So, gwáilóu means “ghostly man” or “devil man” when translated word for word.