I read a lot, and one of the genres I like is the mystery novel. One of the expressions favoured by the English writer Reginald Hill, author of the Dalziel and Pascoe series, is the paradigmatic pattern "one/two/a few X short of a Y", meaning "not all there" in the mental sense. The pattern can be realized in a number of ways, only limited by the inventiveness of the writer or speaker. The screen shows four examples taken from Hill's novels. Another, amusing example is found in a novel by the Scottish science fiction writer Iain M. Banks. The narrator, as one can see, is semi-literate.

one/two/a few X short of a Y = "not all there"

"I never really fancied him, two sheets short of a bog roll, I reckoned." (Reginald Hill, On Beulah Height, 1999)

"Jorge, who was definitely one Ave short of a rosary..." (Id., Arms and the Women, 2000)

"She has a son Donald, who is two leeks short of a harvest supper but is excellent for heavy work around the garden." (Id, ibid.)

"The aunt is a few twigs short of a tree, but she's not a nut." (Id., Good Morning, Midnight, 2004)

Cf. Iain M. Banks:

"Thi golden hare-and-Is bloak dozen seem 2 ½ chainjed sins thi kaos got in2 thi towrs computirs but then frangli he woz a few fevvirs shot ov a fool wing 2 start wif so no chainje thare." (Iain M. Banks, Feersum Endjinn, 1994)