poo-poo
Wave away contemptuously
Poo-poo works as both noun and verb.
As a noun, it's childish or euphemistic for feces: Does baby need to go poo-poo?
As a verb, to poo-poo something is to dismiss it as unworthy of serious consideration: Critics poo-pooed the proposal.
The dismissive verb likely comes from the contemptuous exclamation, not the excrement.