par (prep.) Look up par at
"by, for," mid-13c., from Old French par, per, from Latin per (see per).
par (n.) Look up par at
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.