In Terry Pratchett's Discworld books (Witches Abroad in particular), the character Magrat Garlick is often called a "wet hen" by at least one of her witch colleagues.
Web searches only yielded the phrase [to be] "mad as a wet hen". From the description there I think it could mean that she is looking for her "nest", meaning she is trying to get a man, but I am far from certain.
From context in the books I am pretty sure that it does not mean she is mad. She does actually seem quite sane, for a Discworld character anyway.
Here are some quotes for context:
Even Magrat had one [a witch hat], although she hardly ever wore it on account of being a wet hen.
"That's 'cos you're a wet hen, Magrat Garlick," said Granny.