The expression “mad as a wet hen” means: Very, very vexed.
Though, unlike a duck, a hen does not deliberately seek water in a pond and is not disposed to leave a dry shelter in search of food in the rain, this metaphor has never made good sense to me.
Some of our friends have, at various times, sought small fortunes from chickens (with much of the care devolving upon me), but we have never seen a hen becoming particularly perturbed from getting wet.
If she has a brood of chicks, she will cluck them under the shelter of her wings during a shower, but she certainly doesn’t stamp her feet and rave noisily, no matter how wet she herself becomes.
Yet many a man, woman, and child, with far lesser cause, is said to become “mad as a wet hen.”
Be that as it may, whether sensible or not, the expression has been good American since at least 1823, as determined by a correspondent in American Speech, Vol. XXI: “Every body that was not ax’d was mad as a wet hen.”