When I first heard about growth hacking back in the day, as distinct from typical marketing, it was defined as engineering the product itself to propagate to new users. The “classic” example at the time was building social features into programs - Eg, spotify’s social integration.

Thing is, to the best of my knowledge, it (a) never really grew to anything beyond social integration, and (b) was soon expanded in scope to just mean “all marketing by people who wish they were in Silicon Valley,” and subsequently to “all marketing.”

But it had a real definition behind it at some point.