It's had various adjacent meanings over the years (and I may have embellished a little bit in my original interpretation), but here's one explanation which isn't far off: https://tim.dreamwidth.org/1784423.html

> Also, calling Rust a research language is funny to me because -- as its name reflects -- we've tried hard to avoid incorporating new technology into it. We haven't always succeeded at failing to be novel, but we have a rule of thumb of not including any ideas in the language that are new as of the past ten years of programming language research. The field of programming language is full of old technology that hasn't been put to use in solving problems that it's exactly suited for. The goals with Rust were to avoid reinventing wheels, and see what the past had to teach us.