1620s, "equality," also "value of one currency in terms of another," from Latin par "equal, equal-sized, well-matched," also as a noun, "that which is equal, equality," of unknown origin. Watkins suggests perhaps from PIE root *pere- (2) "to grant, allot," with suggestion of reciprocality.
Another guess connects it with PIE root *per- (5) "to traffic in, sell" (on notion of "give equal value for"); see pornography. Meaning "average or usual amount" is first attested 1767. Golf usage is first attested 1898. Figurative use of par for the course is from 1928.