late 14c., propiciacioun, "atonement, expiation," from Late Latin propitiationem (nominative propitiatio) "an atonement," noun of action from past-participle stem of Latin propitiare "appease, propitiate," from propitius "favorable, gracious, kind, well-disposed." The current explanation of this (as of de Vaan) is that it represents *propre-tio-, from PIE *propro "on and on, ever further" (source also of Sanskrit pra-pra "on and on," Greek pro-pro "before, on and on"), from root *per- (1) "forward," hence "in front of, toward, near." It is thus related to Latin prope "near."
Earliest recorded form of the word in English is propitiatorium "the mercy seat, place of atonement" (c. 1200), translating Greek hilasterion. The meaning "that which propitiates or appeases, a propitiatory gift or offering" is from 1550s.