lustrum (n.)

(plural lustra), "ceremonial purification of the Roman people every five years," 1580s, from Latin lustrum "a purificatory sacrifice, ceremony of purification; five-year period," from Proto-Italic *lustro- "expiation," which is of uncertain origin. Perhaps [OED] from root of luere "to wash" (from PIE root *leue- "to wash"). Or [Watkins, Klein], based on a possible earlier meaning "illumination," from PIE *leuk-stro-, from root *leuk- "light, brightness." De Vaan prefers as most likely the explanation "that lustrum was derived from *luH- 'to set free'," with suffix *-stro- also found in monstrum, etc.