Over on Politics.Meta.SE a comment by user Guest271314 asserts a repugnant etymology:
…You cannot expect readers to parse when you are engaging in direct communication or "colloquially" speaking. For example, "cheaper by the dozen" is a "colloquial" English term that actually refers to plantation owners forcing prisoners of war to impregnate their mother, resulting in a "dozen" "cheap" children with severe birth defects.
– guest271314 Jan 17 at 18:59
On the other hand, Google Ngram has nothing from the 19th century that remotely suggests such an origin. Its earliest Ngram usage is from The Freemasons' Monthly Magazine, March 1844, and refers to copies of a Masonic handbook for sale.
What is the origin of the phrase "cheaper by the dozen"?
Best Answer
Using Eighteenth Century Collections Online, I found this note at the end of an anti-Catholic pamphlet titled A Protestant's Revolution (Dublin, 1734), where other pamphlets by the same publisher ("S. Hyde, Widow in Dame-street") are advertised for publication. At the end of the list appears a nota bene:
The statement appears to appeal to cost in bulk.
It's hard to know when the phrasing became idiom, but this example occurring so early and in the context of selling books suggests that its origins were likely in something as prosaic as what the words literally mean together in a marketing context and not a peculiar plantation breeding program that should be well-documented but yields nothing in the resources I've used so far.