The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with daily dozen
Physical exercise, as in Helen walks two miles every morning—that's her daily dozen. This term originally referred to a set of twelve specific calisthenic exercises to be performed every day. They were devised by a famous Yale University football coach, Walter Camp (1859–1925), and came into general use in the early 1900s. Despite the physical fitness craze of the late 1900s, these exercises and their name are no longer taken literally, but the term survives in a very general way.