Its a common occurrence in various stories that a character who wishes to impress their friends will claim to have a girlfriend living in Canada. Obviously this works since it is far enough away to explain their absence, but close enough to be plausible.
What is the origin of this conceit?
Can this be traced back to having originally occurred in any particular story?
Of the numerous examples from film given by TVTropes, the earliest are from 1969 and 1985:
A prospector in Paint Your Wagon [released October 15, 1969] admits that his girlfriend back home, Lisa, was entirely made up.
Anthony Michael Hall's character in The Breakfast Club [February 15, 1985] claims to have a girlfriend living up in Niagara Falls, in addition to having had sex with several other girls. When pressed, he admits he made the whole thing up because he's embarrassed to admit he's a virgin.
- His character also has a distant girlfriend in Weird Science [August 2, 1985] from later in the same year, 1985.
It's likely the trope caught on after these 1985 films.
