This will partly depend on whether you are interested in first/early English-language usage. I believe you may find the earliest usages in German and Italian, and probably from the mid-19th century, if not earlier. Pareto, certainly (Manuale di economia politica, 1906). You might want to check out (but not for an answer to your question): Joseph Persky, "Retrospectives: The Ethology of Homo Economicus," Journal of Economic Perspectives, Vol. 9, No. 2. (Spring, 1995), pp. 221-231. Mathew Forstater