early 15c., "the giving of medicine (in a specified amount or at a stated time)," from Middle French dose (15c.) or directly from Late Latin dosis, from Greek dosis "a portion prescribed," literally "a giving," used by Galen and other Greek physicians to mean an amount of medicine, from stem of didonai "to give" (from PIE root *do- "to give"). Slang meaning "venereal disease" is from 1914.
1650s, from dose (n.). Related: Dosed; dosing.