a sandwich short of a picnic
jokey colloquial phrase

- said to indicate in a humorous way that you think someone is very stupid or is behaving very strangely

- a pejorative phrase meaning not very intelligent or of questionable mental capacity

Collins Dictionary / The Free Dictionary

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WORD ORIGIN

This interesting appeared first in print in the Philadelphia Daily News on 15th May 1985 in a report about the American magician duo Penn & Teller:

“You watch, you watch wondering all the while if these guys aren’t maybe just a few sandwiches short of a picnic.”

Other “short-of” expressions with the same meaning are: “Three bricks short of a load”, “one card short of a full deck” (of playing cards), “One can short of a six pack” (beer), “One sausage short of a barbecue”.

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THE LIGHTS ARE ON, BUT NOBODY’S HOME

Additional to expressions using “short of”, there are more creative ways to describe stupidity or general craziness:

- his/her lift doesn’t go to the top floor

- he’s/she's not the sharpest knife in the drawer

- he’s/she's not firing on all cylinders

- he’s/she's out to lunch

- He's/she's lost his/her marbles

- He/she hasn’t got both oars in the water

- His/her river doesn’t run all the way to the sea

- The lights are on, but nobody’s home

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OTHER SYNONYMS

- having an unstable state of mind

a bit lacking upstairs, ape, arbitrary, around the bend, as daft as a brush, as mad as a hatter, as mad as a March hare, as thick as two short planks, away with the fairies, bananas, barking mad, barmy, batty, bedlam, bird-brained, blockheaded, bone-headed, bonkers, bovine, brain-dead, bubbleheaded, certifiable, chuckleheaded, clueless, cock-eyed, cockamamie, crackers, crackpot, crazy as a loon, cretinous, cuckoo, daffy, daft, daft as a brush, dead from the neck up, deprived of one’s wits, deranged, dimwitted, ding-a-ling, dippy, doltish, donkeyish, doolally, dopey, dotty, dozy, dunderheaded, feather-brained, feather-headed, feeble-minded, flakey, gaga, goofy, gormless, half-witted, hare-brained, harum-scarum, have a screw loose, have bats in the belfry, have kangaroos in the top paddock, having a screw loose, haywire, imbecile, in another world, in the ozone, kamikaze, kookie, lamebrained, loco, loony, loony tunes, loopy, lunatic, lunkheaded, mad as a hatter, mad as a March hare, madcap, mental, mentally deranged, moonstricken, moonstruck, moronic, muddle-headed, muttonheaded, nitwitted, non compos (mentis), not all there, not quite right in the head (the mind), not right upstairs, not the full shilling, nuts, nutty, nutty as a fruitcake, nutty as fruitcake, oddball, off-the-wall, off your head (your rocker, your trolley), ONE SANDWICH SHORT OF A PICNIC, pea-brained, potty, raving mad, round the bend (the twist),  scatterbrained, scatty, screwball, screw loose, screwy, sheepheaded, slow on the uptake, stark raving (staring) mad, thick, thick as two short planks, thickheaded, tomfool, unglued, unhinged, up the pole, wacko, wacky, with a screw loose, wooden-headed

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SMUGGLE OWAD into an English conversation, say something like:

“He seemed to be SEVERAL SANDWICHES SHORT OF A PICNIC,... that’s why they didn’t hire him.”

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and,

Paul Smith, IBAN: DE75 7316 0000 0002 5477 40