Canucks = French Canadians?
The use of "Canuck" parallels that of some other potentially offensive nicknames, that is, when used by the people it names -- Canadians in this case -- it is usually acceptable. But when used by an outsider -- in this case particularly American strangers -- it can be easily misinterpreted and deemed as insulting one's heritage. Although it is not as severe as most ethnic slurs, some consider it one.
The Random House Dictionary notes that: "The term Canuck is first recorded about 1835 as an Americanism, originally referring specifically to a French Canadian. This was probably the original meaning, though in Canada and other countries, "Canuck" refers to a Canadian."