Hardly anyone went to see the movie The Bucket List but everyone knows what that term means. While I am sure the term predated the movie, it was the movie (or more precisely the marketing for the movie) that put the term into pop culture where it stays. Can you think of other examples where the lasting effect is much more than the movie itself?
Well, quite a few films didn’t do well to start but are now classics; Fantasia, Wizard of OZ, Citizen Kane, It’s a Wonderful Life, Blade runner.
The term was defined in the trailer and the TV commercials, which more people saw than the movie. It’s not like they had to see the film to know what a “bucket list” was.
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Just Imagine A big, expensive flop for its time. Hollywood didn’t do big budget science fiction again for over 30 years and practically no SF for twenty.
Cutthroat Island did the same thing for pirate movies.
I am almost certain that this is exactly what the OP meant, not a counterargument at all.
The Bucket List is a perfect example. I never heard the term before that movie came out, and I thought it was kind of dumb too. And I only ever saw the movie because I couldn’t sleep on a long flight.
I think Donnie Brasco is a fairly good example, too. Thanks to that movie, everyone knows the difference between a “friend of mine” and a “friend of ours”. Also that’s where we learned about “fuhgeddaboutit” with all its nuances.
It seems to me most coworkers under 35 or so religiously quote Office Space, which flopped miserably at the box office. It gained its audience on home video.
I have never heard of the movie The Bucket List, but have heard the term ‘bucket list’ for years.
What’s the difference between “friend of mine” and “friend of ours”?
Maybe not quite what you’re looking for, but there’s tons of songs from movies that permeated popular culture even though the movie was a total flop. For example, the Righteous Brothers’ song Unchained Melody which was written for the forgotten movie Unchained.
And Comedy Central re-runs.
According to its Wikipedia page, Forbidden Planet earned 1.6 million on initial release - not much even in 1956.
But it was the first movie that had humans traveling in space through human technology, and of course it gave us Robbie the Robot.
“friend of mine” - Mob associate or a connected guy.
“friend of ours” - Made guy.
‘Fuhgedabadit.’ Just sayin’… 
Pardon? The aforementioned Just Imagine (1930) has a rocket to Mars. Destination Moon(1950) has humans using a human-built rocket to successfully reach their, umm, destination, the Moon. Or do you mean the first movie with humans using human-built means to cross “deep” or interstellar space?
That makes you unusual. The term, as near as I can tell, does predate the movie, but not by much, and even the OED credits the movie with popularizing it.
Other than movies that are famous for being flops, I can’t think of any box office failures that fit this category. However, it seems to me that the line “Wax on, wax off” is familiar to more people than could possibly have seen The Karate Kid. Also, I think the premise of An Indecent Proposal was discussed at cocktail parties, etc. by more people than ever bothered to see the movie.
Um, yeah, let’s go with that. Interstellar. I just grabbed that from the wikipedia page, although I knew that Forbidden Planet had some general innovative stuff.
Not sure if this fits what the OP has in mind, but for years (many many years ago…) Jack Benny used his famous movie flop “The Horn Blows At Midnight” as a built-in punchline.