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- "to grant, allot," with suggestion of reciprocality (see part (n.)).
Another guess connects it with PIE root *per- "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.
"by, for," mid-13c., from Old French par, per, from Latin per (see per).
n. pl. pa·ri·a (pä'rē-ə)
A pair; specifically, a pair of cranial nerves.
What is to be expected: He had to take a little crap from the clerk, but that's par for the course (1947+)