A Canadian term of both derision and affection, popularized - but NOT invented - by the MacKenzie Brothers (from their "Great White North" segments on the old SCTV programme).
Used as a negative, it basically means "idiot." When used positively, it's a phrase of light endearment.
In both cases, it is most effective when the subject(s) displays stereotypically Canadian attributes.
NEG - "They drank all our beers and smoked all our smokes. What a bunch of Hosers."
POS - "They drank all our beers and smoked all our smokes, but took the ski-doos to get more before the hockey game started. What a bunch of Hosers."