self-negating descriptions

brownbetty asks if there is a name for things you cannot say about yourself without inherently conveying the idea that they are not true. For example, "I'm a classy guy" = you are not a classy guy.

I find this sort of thing endlessly fascinating, as I am fascinated by things which cannot be communicated by saying them! Another example is "Please only do [x] if it is absolutely vital." If you say that, everyone who wants to do [x] will decide it is important enough and will do [x]. If you want people to only do [x] if it is absolutely vital, say "No one can do [x]." People for whom it is vital will do it anyway, because they have no choice.