It is an offensive term to describe someone who is being obnoxious and offensive. Poo is the slang word for faeces.

As Jura says, it is an offensive term, but it's also a childish one: "Poo-poo" is a child's term for feces. The speaker either is a child or is using a juvenile term in order to soften the insult to Eliot.

someone horse's ass or oaf