A limerick containing Dad's favorite phrase

There once was a fellow named Hank
Who said, “Let’s be perfectly frank
“This party’s OK
But, please, don’t say ‘birthday'”
“POO POO!” cried the guests, as they drank

In honor of my Dad, who retains a prodigious vocabulary in his later years, but has added a number of phrases like “poo poo” and “toot toot.” Friends and family members find such words charming in such an erudite individual, but wonder what the words actually mean.