early 15c., prolongen, "lengthen in time, extend the duration of; delay, postpone," back-formation from prolongation or else from Old French prolonguer, porloignier (13c.) and directly from Late Latin prolongare "to prolong, extend," from Latin pro "forth" (see pro-) + longus "long" (adj.); see long (adj.). The same elements also form purloin. Related: Prolonged; prolonging; prolongable.