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Originally and chiefly American English, the colloquial phrase one’s cheese slipped off one’s cracker, also one’s cheese slid off one’s cracker, and variants, mean: one is no longer rational or sane.

For example, Douglas Ford (born 1964), Premier of Ontario, Canada, used this phrase about Donald Trump (born 1946), President of the USA, during a press conference held at Thunder Bay, Ontario, on Thursday 31^st July 2025—source: Premier of Ontario:

This guy will say something one day, and he’ll wake up, and the cheese slips off the cracker, and then all of a sudden he goes the other way, and you’re thinking, how do you deal with a guy like this?

These are, in chronological order, the earliest occurrences that I have found of the phrase one’s cheese slipped off one’s cracker, also one’s cheese slid off one’s cracker, and variants:

1-: From Goodbye, Farewell and Amen, a U.S. television film directed by Alan Alda, and first broadcast on CBS on Monday 28^th February 1983 (this film served as the final episode of the U.S. television series M*A*S*H)—source: IMDb:

Hawkeye: [voice over, as he is writing to his father] Dear Dad, For the first time I understand what a nervous disorder is, because it seems I’ve got one. I guess I’ll be seeing you soon, because I doubt if they’ll let a surgeon operate whose cheese slipped off his cracker.

2-: From Active elderly artists apt to be stimulating, by Dan Huff, published in The Arizona Daily Star (Tucson, Arizona, USA) of Thursday 21^st February 1985 [Lifestyle section: page E6, column 1]:

The entire population of the United States is getting older, and that has East Coast radio producer Connie Goldman intrigued, if not excited.
[…]
Goldman has put together a 13-part series of interviews with folks who aren’t letting the cheese slip off their crackers just because they’re old. “I’m Too Busy to Talk Now: Conversations with American Artists Over 70” premieres at 4:30 p.m. today on KUAT-FM.

3-: From The IRS and the grasshopper, by Mary Lawrence, published in The Indianapolis News (Indianapolis, Indiana, USA) of Tuesday 17^th September 1985 [page 6, column 3]:

“Is this the office of the Internal Revenue Service?”
“One of them. May I help you?”
“I most certainly hope so. I’ve come to declare now that on my next tax form I’m going to change my listing of occupation from ‘journalist’ to ‘grasshopper’.”
[…]
“Ma’am, forgive me for saying so, but your cheese has slipped off your cracker.”

4-: From a theatrical review by Larry Fennelly, published in the Macon Telegraph and News (Macon, Georgia, USA) of Friday 11^th October 1985 [Leisure section: page 2D, column 4]:

The yahoo who reviewed the current show at Macon Little Theatre clearly has a cheese that has slipped off the proverbial cracker. Writing up the show, the reviewer (who, it must be said, stepped in as a pinch-hitter following a dental emergency on the part of the scheduled writer) ignored numerous deserving performers.

5-: From the fourth part of The nameless enemy, about terrorism in the USA, by Mike Bennighof, published in the Birmingham Post-Herald (Birmingham, Alabama, USA) of Thursday 31^st October 1985 [page A7, column 6]:

Some terrorists do not have political goals; they’re just crazy, Baker said.
“Call it what you like. Their elevator doesn’t go all the way to the top; the cheese has slipped off their cracker,” he said. “We can never plan for people like (John) Hinckley. You never know when they’re going to show up. They look just like you and I; you never know when they’re going to strike.” Hinckley was charged with trying to assassinate Reagan in March 1981. He was found innocent by reason of insanity.

The phrase one’s cheese slipped off one’s cracker, also one’s cheese slid off one’s cracker, often occurs in glossarial contexts—for example in the following from the Daily Echo (Bournemouth, Dorset, England) of Saturday 16^th October 2004 [Magazine section: page 59, column 3]:

As smart as bait
Parting shots—ways of describing someone who is intellectually challenged

● Not the sharpest knife in the drawer.
● The lights are on, but nobody’s home.
[…]
● A few sandwiches short of a picnic basket.
[…]
● He only has one oar in the water.
● A few beers short of a six-pack.
[…]
● All foam, no beer.
● The cheese slid off her cracker.