I'm searching for British English expressions describing a person who starts to be stupid, crazy or foolish. I mean something like the idiom to lose one's head and epithets like: You fool! Are there any where animals are used?
For an emphasis on the progression of cognitive impairment, there are lots of "going" and "losing" phrases.
The most animal-related one I can think of that is used e.g. for the increasingly-senile elderly is to go batty. Dictionaries seem split on this: though the key element they all agree on is an aspect of mental instability or madness, several also include the notion that a batty person is eccentric or unpleasant, which seems important in the senses I've seen it used ("batty old cat lady", for instance). According to the Oxford Dictionaries, the "bat" in "batty" doesn't actually refer to the flying noctural mammal, but to a second sense of "bat", an unattractive or unpleasant woman, that may be derived from French bat (prostitute) or perhaps from "battleaxe". Note that "batty" has a completely different meaning in Jamaican slang, related to backsides and hence to homosexuality.
Go dolally or indeed doolally, doo-lally or doolali is from British Army slang for the Deolali sanatorium, Marashtra, India. Originally an insane person was said to have the "dolally tap" (with various anglicisations of the place-name) but while I have often heard "go dolally" I have never heard the original "tap" form, which is certainly archaic. (The linked source says that even "go dolally" is archaic, but I have heard many older people use the phrase, and younger people seem to understand it.)
There is also go round the bend for loss of sanity (from exasperation, intoxication or decrepitude) or, with more emphasis on eccentricity than insanity, go round the twist. To go loopy can capture the crazy eccentricity of "round the twist", or a sense of confusion and befuddlement.
Go off one's head/go out of one's head indicates that someone has become mad or demented (and "off one's head" is often used for states of intoxication, e.g. "off his head on LSD"). There are related terms go off one's rocker or off one's trolley or off one's nut which, in my experience and from a few internet searches, are much more rarely associated with intoxication and more often to cognitive impairment from age.
We also have losing it and variants such as losing the plot, losing one's mind, losing one's marbles, losing one's senses. Note that "losing it" is very general, and often means losing one's temper or emotional control. But it can also refer to a reduction in aptitude or skill, which is useful if you want to emphasise the loss of practical intelligence: "Tiger Woods has lost it", for example.
If you need an animal involved:
- Try go cuckoo. Or go cuckoo in the head.
- Another one is go batty. See http://www.phrases.org.uk/meanings/bats-in-the-belfry.html for origins.
- Bats in the belfry as in "She's got bats in the belfry."
- Squirrels in the attic.
- Mad as a March hare.
- Owls in the loft.
1. a stupid person.
2. a male ass or donkey.
Being called “as dumb as a doornail/as dumb as a doorknob” originally meant that one was silent/unable to talk or make sounds, but like the meaning of “dumb,” the meaning of these two expressions has expanded to include “[being really] stupid” (from LakeRegionHome[dot]com, a Real Estate blog).
(The “doorknob” version might work better for “becoming stupid,” because you could easily (and almost cleverly) add “turning [as]” to it to hint at the “becoming” aspect: “s/he’s turning [as] dumb as a doorknob.”)
Although I'm not sure if this is used in British English, for an animal version of this expanded meaning of “as dumb as,” there’s “s/he’s [becoming] as dumb as an ox” or simply "You dumb ox," although I understand that oxen are actually quite intelligent. (from “Ox-team Days on the Oregon Trail” taken from Google Books)
As mad as a hatter is originally a British English expression, Phrase Finder says
Mercury used to be used in the making of hats. This was known to have affected the nervous systems of hatters, causing them to tremble and appear insane. … Whilst not being the source of the phrase, we can't mention 'as mad as a hatter' and leave out Lewis Carroll. His 'Hatter' character from Alice's Adventures in Wonderland, 1865, is of course the best-known mad hatter of them all. and he is portrayed as mad (along with all the other characters) by the Cheshire Cat:
I've always liked the silly expression to be potty. It is a quintessentially a British English term, and means a person who is often so obsessed about a particular thing it makes them appear either eccentric, a buffoon or a lovable fool.
This one I copied from a text book lying near me which discusses about otherwise promising athletes who start to lose their mojo and then disappear into obscurity. I don't know if this is a bit extreme for you.
According to merriam-webster:
Definition of mojo: a power that may seem magical and that allows someone to be very effective, successful, etc