According to Bob and Doug McKenzie, "hoser" is "what you call your little brother when your mother's in the room." It's a fairly mild insult, and in normal usage it refers to a beer-drinking, hockey-obsessed, none-too-bright Canadian male, but it's also used as a blanket slur for all Canadians.

Perhaps you are too young to remember Bob and Doug McKenzie (they're still around, but not quite the sensation they once were). Played by Dave Thomas and Rick Moranis, Bob and Dog were dim-witted Canadian brothers who hosted a show called "The Great White North" in a popular recurring SCTV sketch. These bits on SCTV spawned an album and the movie Strange Brew, a cult hit north and south of the border. Bob and Doug played up traditional Canadian stereotypes and made frequent use of the word "hoser." Viewers would soon follow their lead.

It's largely because of Bob and Doug McKenzie that "hoser" gained popularity in the '80s, but they didn't invent the term. The etymology is not clear, but it appears to be an old Canadian slang term dating back to at least the '30s. The most likely explanation I've seen was that "hosers" and the related "hoseheads" were originally Depression-era farmers in western Canada who would steal gasoline by siphoning it off from tanks with a hose, which required sucking on one end of the hose and thus often left the hoser with a mouthful of gasoline. (This covers "Canadian" and "not too bright.")

This is not the only possible origin, however. "Hoser" could have been derived from other slang uses of "hose." For example, "hosed" is sometimes used as slang for "drunk" ("I was so hosed last weekend!"); "to hose" is a verb meaning "to cheat" or "to put in a very unpleasant position" ("We got hosed on that exam!"); and "hose" also falls into the category of slang terms describing the male genitalia, many of which are used as insults (this would explain "hosehead"). But the gas-siphoning explanation seems more Canadian in origin and makes more sense in terms of usage.

Sources: Wikipedia: Hoser, HomeEnvy, American Dialect Society Mailing List, Origins of Racist Terms