According to Cocker
Bywords, according to one definition, are proverbial sayings that express some important fact of experience that is taken as true by many people. For example: had you lived in the 17th-18th centuries, and you had wanted to convey the idea that something was "absolutely correct" or "according to the rules," you might well have ended your assertion with the phrase "according to Cocker."
What "important fact of experience," "taken as true by many people," would have led you to end your assertion with this phrase?
Edward Cocker (1631-1675) was an English engraver, writing master and mathematician whose magnum opus, Cocker's Arithmetick, was published posthumously in 1677. Over the next 150 or so years, this title (which contains the earliest known use of the concept of lowest terms) educated generations of British schoolchildren. (The volume depicted below is the 33rd Edition of 1715, from the collection of Augustus De Morgan held by the Senate House Library at the University of London:)
Much of this title's popularity and influence is attributed to the fact that it excluded all demonstrations and reasoning, and confined itself to commercial questions only. It is to the presumed accuracy of Cocker in resolving commercial mathematical questions (since disputed) that the phrase "according to Cocker" arose as a byword for "absolutely correct."
Samuel Johnson carried a copy of Cocker's Arithmetick on his travels about Scotland, and the book was widely used in colonial America, not least by folks like Benjamin Franklin. It would be an interesting collecting challenge to try and obtain as many editions of this title as possible, although several editions (DNB suggests at least 112 editions may have been published altogether) do not appear to have survived in even a single copy. As a grammar schoolbook subjected to generations of hard use, this is to be expected. Makes the challenge more interesting...no?