I found the word in Heinlein's "Friday":
I apologized, saying that there was no excuse for me to be sleepy since it was still early evening by the zone where I had started the day.
Janet said pishantosh, that being sleepy had nothing to do with clocks and time zones -- gentlemen, we are going to bed. She led me away.
I think, I can grasp the meaning from the above context, but I'm surprised, search-engines return nothing useful for it. What does the word actually mean and where is it coming from?