(adjective/adverb) - According to Cocker is equivalent to "sure to be right." In the days of the Stuarts lived a man, Edward Cocker. In his day and a long time after, his work on arithmetic was in general use in this country, and regarded as a standard of accuracy.
"Something done according to Cocker was done properly, according to established rules or what was considered to be correct.
The etymological story starts in 1678, when John Hawkins published the manuscript of a book which Edward Cocker had left at his death two years earlier. Cocker had been the master of a grammar school in Southwark, across the Thames from the City of London, and Hawkins was his successor in the post. (It has been claimed that the book was actually by Hawkins, trading on Cocker’s name, but the current view is that Cocker really had written it.) The book, after the fashion of the time, had an expansive title — Cocker’s Arithmetick: Being a Plain and familiar Method suitable to the meanest Capacity for the full understanding of that Incomparable Art, as it is now taught by the ablest School-masters in City and Country."