I would hesitate to call myself an MAist but plenty of people I know (mainly in Dan ranks) have said to me that they would call me so because of my attitude. To really drill down to the meaning would be difficult, to pin it exactly, impossible. We really are going to come down to a large area of opinions about the meaning, but it'd be good to witness that area reduced as much as possible.
Is it just training or and application that make one a martial artist, or is it a mind set?
These points of view also are on the same page, and I think they dovetail nicely with the previous ones, though not in completely obvious way. I think both of you are talking about something that I'd try to capture as a kind of seriousness about what you're doing. Proficiency flows from seriousness (in the fullness of time, of course!) so although they aren't quite the same thing, they're related. For some people I've worked with, TKD is kind of an incidental part of their lives; it's not an endeavor that enters into their view of who they are. They do it, but it's not something they think about, or try to work on to consciously improve ... in other words, they don't take it seriously past a certain point. People I think of as MAists think about what they're doing when they do TKD (or whatever art); it matters to them how a certain technique works or what a certain hyung is telling them about combat. And of course, such people do tend to get more proficient much faster.
Shads is right: we can only narrow the meaning down so much before we hit the level of irreconcilable individual variationwe're going to wind up with a circle, not a point, because `MAist' is fuzzybut it's good to try to make the radius of that circle of variation as small as possible...