A traditional abbreviation for 1/2, from the Latin semis. In medieval times the symbol was often written as the long s, an obsolete character that looks something like f without the crossbar
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}