"desecrate, treat (holy things) with irreverence," late 14c., prophanen, from Old French profaner, prophaner (13c.) and directly from Latin profanare (in Medieval Latin often prophanare) "to desecrate, render unholy, violate," from profanus "unholy, not consecrated" (see profane (adj.)). Related: Profaned; profaning.
mid-15c., prophane, "un-ecclesiastical, secular, not devoted to sacred purposes, unhallowed," from Old French prophane, profane (12c.) and directly from Latin profanus (in Medieval Latin often prophanus) "unholy, not sacred, not consecrated;" of persons "not initiated" (whence, in Late Latin, "ignorant, unlearned"), also "wicked, impious."
According to Lewis & Short, de Vaan, etc., this is from the phrase pro fano, literally "out in front of the temple" (here perhaps with a sense of "not admitted into the temple (with the initiates)," from pro "before" (from PIE root *per- (1) "forward," hence "in front of, before") + fano, ablative of fanum "temple" (from PIE root *dhes-, forming words for religious concepts). The sense of "irreverent toward God or holy things" is from 1550s. Related: Profanely.