"Jook-sing" means a grain-measuring container made of bamboo (compare the term 升斗 (jyutping: sing1 dau2), being a kind of rice measurer). Bamboo is hollow and compartmentalized, thus water poured in one end does not flow out of the other end. The metaphor is that jook-sings are not part of either culture: water within the jook-sing does not flow and connect to either end. It may or may not be derogatory. Use of the term predates World War II.
In the United States and Canada, the term is pejorative and refers to fully Westernized American-born or Canadian-born Chinese. The term originates from Cantonese slang in the United States. Jook-sing are categorised as having Western-centric identities, values and culture. This term also refers to similar Chinese individuals in Australia, Malaysia, Singapore, and New Zealand.
Banana (Chinese: 香蕉人/香蕉仔; pinyin: xiāngjiāo rén / xiāngjiāo zi; Jyutping: hoeng1 ziu1 jan4/hoeng1 ziu1 zi2) (referencing the yellow skin and white innings of the fruit when fully matured) and Twinkie (based on the snack produced by American company Hostess - again, it denotes something that is "yellow" on the outside and "white" on the inside): often pejorative.