Fred R. Shapiro, the editor of the Yale Book of Quotations, has shown that in 1952 the adage was called “Murphy’s law” in a book by Anne Roe, quoting an unnamed physicist:

There were a number of particularly delightful incidents. There is, for example, the physicist who introduced me to one of my favorite “laws,” which he described as “Murphy’s law or the fourth law of thermodynamics” (actually there were only three last I heard) which states: “If anything can go wrong, it will.”