Where does the term "Homo economicus" originally come from? Who was
the first to use it?

Hi tor and thanks for the question.

The term  "Homo economicus"  originally comes from the `minimax
theorem'  developed by Von Neumann in 1928.


“And game theory is far from a new science; development began as early
as 1910, and by 1928-the year in which Von Neumann presented his
`minimax theorem' and had coined the term `homo economicus'- it had
attracted the attention of economists who saw in it the potential for
an umbrella theory for the social sciences (Aumann 1986: 461). “
http://216.239.35.100/search?q=cache:iUrcCq43S2kC:www.lafferty.ca/writing/gt-thesis.pdf++%22term+Homo+economicus+%22++&hl=pt&ie=UTF-8
**NOTE** available only in Google cache.

Bio of von Neumann, John(1903 -- 1957) 
http://www.dam.brown.edu/people/yiannis/neuman.html

A description of the 'minimax theorem'  
http://library.thinkquest.org/26408/math/utility.shtml?tqskip1=1&tqtime=0819



There is also a reference which claims that, “Texas historian, T. R.
Fehrenbach, coined the term “
However I am inclined to believe it was Von Neumann in 1928 as T. R.
Fehrenbach delivered a lecture on April 5th of this year(2002) at the
"High Five Ranch Pavillion" on "an historical perspective of ranching
in South Texas."
http://www.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=001NbN

Thank you for the question, and if you need any clarification do not
hesitate to ask.

THX1138

Google search term:
"term Homo economicus"