Green’s Dictionary of Slang

(US) rubbish, nonsense.

1911		G. Bronson-Howard Enemy to Society 314: ‘Oh, pish-tush, Bates!’ returned Canby good-naturedly.
1919		Wash. Herald (DC) 28 Oct. 2/2: Such is the pish-posh on which this comedy is based.
1922		Ogden Standard Examiner (UT) 4 June 4/1: Theater-goers have been treated to an unparalleled assortment of theatrical pish-posh and rot.
1933		C. Himes ‘Prison Mass’ in Coll. Stories (1990) 160: Signifier’s lips curled in a sneer. Just a lot of pish-posh, all of it.