The term "killer app" goes all the way back to the 80s. Back then, Lotus 1-2-3 was considered a killer app as was Wordperfect/Wordstar depending on your needs.
As others have pointed out, it was also used as the file extension for "application programs" on NeXTStep (.app). It continued to OS X which is where "App" came from for iOS applications. It has nothing to do with the name "Apple".
As others have pointed out, it was also used as the file extension for "application programs" on NeXTStep (.app). It continued to OS X which is where "App" came from for iOS applications. It has nothing to do with the name "Apple".