Similar to "Pish Posh" which could be offensive. When modified to "Pish Pash", friendly banter is intended and expected!
NB - The words "Pish Posh" draw the face into an angry expression where "Pish Pash" leaves a smile on the face of the speaker!
It's going to snow all day!
Oh Pish Pash, it'll stop soon and everything will be sunny again!