Having six pet chickens, lots of these little sayings have become clear to me...like "chicken scratch" handwriting and being "cooped up" (chickens don't like it and will raise a fuss if they're used to running around) and "flew the coop" (that's what they do when you finally let them out of the coop...fly for a few feet, they're so happy.) Being cooped up really must be uncomfortable for them, because they like getting out so much!
But mad as a wet hen is still a mystery to me. My poor chickens will just stand out in the rain wondering what on earth is happening to them. They look down, depressed and confused. I literally gotta go out there with a piece of bread and coax them back to their coop so they'll stay dry! Heh heh!! They're so funny.
I think the original saying was something like- mad as a wet setting hen- hen on nest can't get off while it's raining, so all she can do is sputter! Make mine extra crispy!