serum n.

1670s, "watery animal fluid," especially the clear pale-yellow liquid which separates in coagulation of blood in wounds, etc., from Latin serum "watery fluid, whey." This is held to be from PIE *sero- "flowing, liquid," from verbal root *ser- "to run, flow" (source also of Greek oros "whey, watery parts of curdled milk;" Sanskrit sarah "flowing, liquid," sarit "brook, river"). The word was applied by 1893 to blood serum used in medical treatments.