one fry short of a Happy Meal
Adjective
Meaning
(slang) mad, deranged, insane
(slang) stupid, retarded, slow
Source: en.wiktionary.orgSynonyms
not playing with a full deck
few sandwiches short of a picnic
one brick short of a full load
few cards shy of a full deck
a few fries short of a Happy Meal
three fries short of a Happy Meal
Examples
He's one fry short of a Happy Meal.