I always understood the term "full stack" to refer to a developer who has tried (and mastered some of) many different technologies and roles, including embedded, system, sysadmin, backend, desktop frontend, web frontend, DB stored procedures... It doesn't necessarily mean that such a person would be immediately proficient in a given technology (that depends on their recent experience), but given a few months' time they would produce solid code in any of these areas, and within a year they would be proficient in it.
In my understanding it has more to do with a person's ability to learn, or lack of fear from learning, if you wish.
Of course I have met my share of "full stack" devs (PHP + JS), but that's the same as with "senior" devs... Having a job title and being one is not the same thing.
Yeah, I consider myself "full stack" because I actively learn different types of software development that are outside of my current role, which is web front-end and backend (mostly Go and Postgres). I am proficient in s variety of languages, platforms, etc (e.g I love messing with embedded devices), and prefer to advertise my flexibility over whatever area I feel I am best at.
I've met quite a few who are happy specializing and essentially restricting themselves to a couple of technologies. When they call themselves "full stack", I get a bit annoyed, since I feel it should mean more than that.
So yeah, if you give me a month, I'm sure I can become productive in any given stack, and given a year, I'll be mostly indistinguishable from a specialist.
In my understanding it has more to do with a person's ability to learn, or lack of fear from learning, if you wish.
Of course I have met my share of "full stack" devs (PHP + JS), but that's the same as with "senior" devs... Having a job title and being one is not the same thing.