I think based on the comments though, in this case "rat" is purely anyone that is at a higher rank that can't aim as well as the one claiming they're a rat.
Which as my comment states it isn't really accurate since players can actually be ranked higher based on intelligent decision making, instead of just being mechanical aimers.
I'm under no illusions that my aim is good - but I've come from a long career of playing online FPS games to know where the best tactical positions are, how to reach them, and when to engage against enemy teams, where as my teammates are repeatedly more intellectually along the lines of cavemen; "SEE ENEMY. RUN FEET. GUNS BRRR"
And then they have the audacity to blame someone else because they aped into a building solo against three players after giving up the "god spot".
EDIT:
FYI I don't also see myself as a rat unless I'm a solo survivor of my squad. I know that I don't stand a chance of winning a 1v3, so I tactically await teams to engage each other, and then I usually shoot a few bullets to get a knock in the fight on the team that's mostly in cover.
I had to educate my team to a win the other day with the same concept - never shoot the exposed team. You just alert them to your position, and allow them to re-position. Equally, the team already in cover gets alerted to your position, and you're in a stalemate there for first.
Always shoot the team in cover and help the exposed team. They won't adjust their positioning to your incoming fire because you're not shooting them, leaving you (or them) to kill the team that has the advantage, leaving them completely exposed when it becomes your team vs their team.