Why is it called 'business logic'? Where did this term come from?

For the same reason that the end of a gun which the bullets come out of is called the “business end”. It's where the primary action happens.

If you were a professional graphic designer, there certainly would be business involved with using your imaging application - your job is your business!

So "business logic" refers to the parts of the code that define how the user conducts his business (in this case, manipulating images).

Don't forget that back in the day, all software was "business software" - no-one could afford the expensive equipment and skills required to write software for anything other than business purposes. It if didn't make money or save money for a business, it didn't get written.