Racist expression which used to be commonly uttered by Canadian-anglophones towards francophones, especially in Montreal, Canada. It implied that speaking French somehow made the speaker akin to be a person of color (because non-white people would often speak foreign languages), which was an insult at the time.
It was also an arrogant way to remind the francophones of the disparity that existed between the the two communities (the anglophones being in general wealthier and occupying positions of power within the society).
French Waiter : "Bonjour Monsieur, que puis-je pour vous aujourd'hui?"
English Customer (spitefully): "Come on, speak white!"