The term was coined in the 19th century, although its etymology is unclear. Possibilities include:
"Canuck" is a nickname for the Curtiss JN4 and Avro CF-100 aircraft.
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.
One of the first uses of "Canuck" -- in the form of "Kanuk"-- specifically referred to Dutch Canadians as well the French.
"Canuck" also have the rare derived meanings of a Canadian pony and a French-Canadian patois² (very rare).
A Canuck Avenue exists in Toronto.
See also: Yankee, a nickname for "an American".External links
Reference
¹ The Oxford Companion To The English Language
² Oxford English Dictionary