The English word dozen comes from French douzaine, which in turn comes from Latin duodecim: “two plus ten.” It occurs in several English idioms.
1. cheaper by the dozen: costing less when bought in quantity.
The expression appears on the Google Ngram Viewer in the 19th century, but its usage rises significantly beginning in 1942. The novel Cheaper By the Dozen by Frank Gilbreth, Jr. and Ernestine Gilbreth Carey was published in 1948, and the popular movie version based on it (starring Clifton Webb and Myrna Loy) was released in 1950.
2. a baker’s dozen: thirteen instead of twelve
In the Middle Ages, bakers were discouraged from cheating their customers by strict weights and measures laws. Lacking precise modern equipment, medieval bakers couldn’t be sure that every loaf that came out of their ovens would be identical in weight. To protect themselves from being fined, they threw in a little extra with each order.
Two other expressions used to mean “thirteen” are devil’s dozen and long dozen.
3. by the dozen: in large quantities, not necessarily in units of 12.
For example, the following headline uses the expression to mean “in large quantities:
“Applications by the Dozen, as Anxious Seniors Hedge College Bets”
4. daily dozen: a short set of daily physical exercises, performed in sets of 12.
The phrase originally referred to a set of 12 callisthenic exercises devised by Yale football coach Walter C. Camp (1859-1925). The Google Ngram Viewer shows the phrase peaking in 1928 and then dropping off.
5. to talk nineteen to the dozen: to talk endlessly or at great speed.
The fast talker says nineteen words for every twelve that a person speaking at normal speed would say.
Although not an idiom, a commonly heard expression is “the Dozens.”
6. the Dozens: a game of spoken words between two contestants in which participants insult each other until one gives up. The insults may refer to the other player’s intelligence, appearance, and character, or to family members, particularly the other person’s mother.