"Cuss" in this poem refers to "an annoying or stubborn animal."
"Oh, say, what is this fearful, wild,
"This creature (don't say 'cuss,' my child;
'Tis slang)--this creature fierce is styled
His curious name derives its source
From two Greek words: hippos--a horse,
The river's plain enough, of course;
But why they called that thing a horse,
That's what is Greek to me."
Written by Oliver Herford (born in England but moved to the US as a teen)