A play on the phrase baker's dozen; it is one less than a dozen as compared to one more. See also bankers' hours.
banker's dozen (plural banker's dozens)
- (idiomatic, uncommon) Eleven
- (banking, finance, idiomatic) By analogy, a method of lending where the interest is deducted beforehand, archetypally borrowing twelve dollars (owing twelve dollars back, a dozen) but actually receiving only eleven.