I hear people use this word kind of often, and I know it’s negative.
Pretending to be something/someone you’re not.
It’s very like the term poseur, except it can also mean that the person pretending has no business even attempting to be mistaken for what they are trying to seem to be. If that makes sense.
In one context, pretending to be more hip/cool/fabulous/etc. than one is. ‘He’s not all that. He’s a poser (or poseur).’
A quickie definition: Part time punks.