1670s, "watery animal fluid," from Latin serum "watery fluid, whey," from PIE verbal stem *ser- "to run, flow" (source also of Greek oros "whey;" Sanskrit sarah "flowing, liquid," sarit "brook, river"). First applied 1893 to blood serum used in medical treatments.
)window.location='https://www.paypal.com/cgi-bin/webscr?cmd=_donations&business=byronic106@yahoo.com&lc=US&item_name=Donation+to +Help+Keep+Etymonline+Free+and+Open&no_note=0&cn=&curency_code=USD&bn=PP-DonationsBF:btn_donateCC_LG.gif:NonHosted'){kind=link}