If you like keeping your temper, then there’s little chance you’d like to be madder than a wet hen. This American expression, which possibly originated in the Appalachian Mountains, refers to the tempestuous temper of chickens that might accidentally find themselves in water. According to some accounts, chickens get extremely angry if they have to try to swim or fly out of water, since they do neither of these things well. Don’t expect your pet chicken to be anything but furious if you propose a trip to the lake or a visit to a nearby swimming pool.
There are several variants on the phrase. It may be written or spoken as "madder ‘n a wet hen," or "madder than an ole wet hen." The latter phrase wouldn’t make a great deal of sense, since hopefully an “ole” hen, would know not to attempt swimming if it could possibly avoid it.
Though the phrase seems to originate in the American South, it appears to proliferate through the Midwest too. It seems especially common in rural and farming communities, or previously rural communities where no doubt the behavior of chickens is common knowledge. The phrase implies something more than furious though. If the hen is mad, you’re even madder, so you are in fact furious.
To be madder than a wet hen shouldn’t be confused with phrases like mad as a hatter. In the second phrase, mad refers to crazy, not to angry. The first is specifically about temperament and not about sanity, and the use of mad to mean angry is more American than British. People in the UK are not familiar with the phrase and might interpret it to mean that you are feeling very nutty indeed.
In any case, avoiding such situations where you might get too angry is likely a good rule of thumb. There are actually a number of expressions that derive from the behavior of chickens, such as putting all your eggs in one basket, fussing like an old hen, ruling the roost, and getting up with the chickens. Doubtless if you do get madder than a wet hen, people are likely to be walking on eggshells around you.