Lost the plot.

Losing their marbles.

Go/going nuts
Going mental
Going mad
Losing it.
Losing (your/their/his/her) marbles
Losing (your/their/his/her) wits
Going crazy

Oh and

Cracking up
Going insane

Bananas, bonkers, over the edge, loony, loony-tunes

They gone cuckoo! ;¬}

Going round the bend.

Starting to hear voices.

…...
Some words:
......
nutcase, psycho, maniac, lunatic.

We sometimes say ’ gone doolally’. I don’t know the origin of this.

A few bricks short of a load.
All the chairs aren’t under the table.
The elevator quit running to the top floor.

Lost touch with reality

Gone daft in the head

Taken one too many blows to the head

Not riding on all axles

Not firing on all pistons

He’s a real whack-job

He’s wacked out

Nutsy Fagin (I heard Frank Burns on M*A*S*H use that term. I think it had something to do with the character Fagin in Oliver Twist)

One piece of bread short of a sandwich

Not playing with a full deck

A few knives short of a table setting

He’s off to see those men in the white coats

He’s a real section 8

Bat shit crazy

Hearing voices. Losing reality. In his/her own little world.

Mad as a bag of cats
Stark raving mad
Seriously though… Losing touch with reality
On the edge
Has retreated inwards

How could I forget this one, after having just watched Alice in Wonderland last night?

Mad as a Hatter.

Losing their Religion

touched/touched in the head
the lights are on but no one’s home
mad as a hatter
looney tunes
gone batty
lost his wits

A few degrees out of plumb

Speaking as a person with mental illness, I find these euphemisms offensive. I would hope you could find a more respectful way of saying what you want to say. Of course, if this is a joke, then never mind.

There are ways of talking about mental illness that are respectful and accurate. And if you aren’t talking about mental illness, but a sort of comment on the person’s ideas or ways of thinking or eraticism, then why don’t you describe those things specifically instead of resorting to an offensive cliche.

No one can honestly answer this question without understanding the context and message of your poem. I think it is pretty irresponsible of you to just raise an opportunity for people to bash the mentally ill and get away with it. It’s like asking, “what’s another term for nigger?”

@wundayatta And as a person with a mental disorder, I find the term “mental illness” offensive. But none of us get to be The Decider for what’s appropriate language. But for a poem, the OP probably isn’t trying to give a DSM account of mental issues – and I have no issue with “going nuts” or the like, if it actually fits. I much prefer it to tiptoeing around what you actually think of me whilst calling a judge to have me committed against my will.

Hi @wundayatta I don’t think anyone is saying these terms are appropriate, but the question asks for a term that says how to say on the brink of insanity for a poem. The provision of these terms is not a reflection of how I (and I would guess anyone else here) feels about people who are suffering with any form of mental illness or condition.

@MyNewtBoobs Why do you find “mental illness” offensive? What term do you prefer?

@wundayatta Mental illness is, at best, incorrect most of the time. For bipolar, it’s a disorder, not a disease or illness (the organic, genetic part – it’s a bit different with syphilis). But mostly, illness seems to say that you’re sick, which you aren’t. Most mental disorders you simply think and feel differently than the “normal” population does, but you aren’t sick, you aren’t wrong, you aren’t twisted. Mental illness suggests that you should spend all your time popping pills and trying to get better and be normal; mental disorder says “It’s different, I work with it, now GTF over it.” Disorder aims to create a wider circle of tolerance, whereas illness does the exact opposite – it stigmatizes and excludes. Course, if you have a mental disorder or illness or whatever you want to call it, you can call it whatever the fuck you want to. And I’m not hugely offended by it most times, unless people have given me a reason to think that they aren’t just saying a word and my issues with “illness” are their opinion at large.

Use an original metaphor.

Is there a mental disorder/disease/condition/syndrome identified that requires the victims to crap all over lighthearted banter?

@6rant6 No, that’s something some people do.

They went cuckoo for Cocoa Puffs! Although that isn’t a very nice expression for poetry.

I ended up using “losing his wits”. Thanks for the input everyone!

@wundayatta I’m sorry I offended you; I didn’t mean to in any way. I was just trying to find a way to exemplify the way that Ralph, Jack, and the other boys from Lord of the Flies crack under the pressure of survival and lose their sense of humanity and common sense.