1650s, "forecast of the probable course and termination of a case of a disease," from Late Latin prognosis, from Greek prognōsis "foreknowledge," also, in medicine, "predicted course of a disease," from stem of progignōskein "come to know beforehand," from pro- "before" (see pro-) + gignōskein "come to know" (from PIE root *gno- "to know").
An earlier form in the same sense was pronostike (early 15c.), from Medieval Latin pronosticum. The general (non-medical) sense of "a forecast of the course of events" in English is from 1706. A back-formed verb prognose is attested from 1837; the earlier verb was Middle English pronostiken (c. 1400), from Medieval Latin pronosticare. Related: Prognosed; prognosing.