This my definition, but other will disagree.
In the simplest terms: It is when you are barely doing the with bare amount of effort and not taking the extra initiave.
Extra initiative could be mentoring, hiring, comittees, technical talks, participating in technical RFC's, hacking side projects.
A coasting engineer would be a Subject Matter Expert but will avoid the extra initiave. This could be for number of reason, but the main one I see is older Engineer (40+) rather spend time with their families then waste their life on more meetings. Myself I stopped doing those because I was looking for another job lol.
At FAANG this is a grave sin for some (was for Zuckerbot especially). You are suppose to go above and beyond and pave the path for the future of the company.
Non-FAANG its not an issue as much. I had senior engineer in their 50s struggling with Python (TBF they knew the legacy system very well). The company was doing amazingly, so all the performance of low performing engineer did not matter at all. Unless they were wildly incompetent to point where nothing was delivered, they would stick around. They did not participate in any of the extra initiave I mentioned, but TBF they were not getting spectacular raises either.