This is the 13487th most frequent Polish word.
pchać
"To push."
Here, 'pchać' refers to the physical action of pushing something.
Muszę pchać wózek z zakupami, bo ma zepsute kółka.
I have to push the shopping cart because it has broken wheels.
In this context, 'pchać się' is idiomatic and means to meddle or interfere.
Nie pchaj się w nie swoje sprawy.
Don't interfere in matters that don't concern you.
Here, 'pchać się' refers to pushing oneself forward in a line.