"Pish and posh!" is a nonsensical phrase used to indicate that something is... well, nonsensical. "Pish-tush!" is also used.

I'm not sure. Those words are more British English. So if you are planning to go to America, those words won't be very useful. I have heard the phrase 'Pish! Posh!' before, and it is used to say that something is completely false. Therefore, my guess is that the words mean 'garbage'.