While it is not directly on the history of the term economics, I have a couple of papers that have explored the topic. You might take a look at my paper, "What Was "It" that Robbins was Defining" available to download through Repec http://ideas.repec.org/p/mdl/mdlpap/0706.html

Political economy was used through the 1800s as the broad term of what economists studied.  They discussed the pure science of economics--sometimes called positive economics--which at the time was primarily deductive, because they didn't have the tools to empirically measure the variables, and separated that from political economy.   JN Keynes's book on Scope and Method is a good reference. Marshall started a different usage in part because he was trying to make economics a separate trypo at Cambridge, and called it economics (my suspicion is because of internal political reasons). In doing so he was downplaying the pure deductive science of economics, and instead seeing his "one thing at a time" method" engineering approach to economics as the science of economics. His discussion of the art of economics--Keynes' terminology for the policy branch of economics--changes over the editions, as he gets the separate field of economics established.

Pigou, who followed Marshall, called his and Marshall's science of economics "fruit bearing" or "realistic" science, which he differentiated from pure science. When Robbins wrote his famous piece on the definition, he was clear that he was defining the "science of economics" (which he interpreted in the classical way--very narrowly and primarily deductive) which he contrasted with political economy. See both Robbins' piece and his Ely lecture where he called for the return to the use of political economy.  We have lost that distinction, which leads to much confusion about the ideological content of the field.

David Colander