Oh, the blithery, blathery pirate
(His name, I believe, is Claude),
His manner is sullen and irate,
And his humor is vulgar and broad.
As with many of Silverstein poems, he hits us with some  nonsense words –in this case,  blithery , in order to create a  pararhyme  with  blathery , a term used to describe trashiness or deception.
The Pirate