This review of Mario Kart: Super Circuit from 2001 seems to call the game a "killer app" for the GBA (end of the page). But yeah I've never seen any normies use that term until the 2010's
"killer app" was an awful game marketing term that has been around since the 80s. it was the type of thing you'd hear from some marketing fag at a press presentation like E3. the first time I personally saw it was on the fucking box quote for the original Halo, back in 2001:
the term "app" as shorthand for "application" has been around about as long. "application" itself is short for "application software" which specifically describes user programs like word processors, games, etc. as opposed to system software (the OS) or utility software (disk defraggers, anti-virus, etc). "program", "software", and "application" all technically describe different things; "program" specifically describes a set of instructions in code, "software" is any collection of programs and data that allow the user to perform tasks, and "applications" are those pieces of software the user directly interacts with. in practice, however, these terms are informally interchangeable.
as far as common vernacular goes, though, "app" entered the public consciousness in its current form through the marketing for the iPhone. thus most normies understand that any kind of thing that makes your computer emit colors and sounds is an "app" which, while gay, is not technically incorrect. in fact, in most common uses, "app" is more technically correct than "program" or "software".
