compare v.

word-forming element usually meaning "with, together," from Latin com, archaic form of classical Latin cum "together, together with, in combination," from PIE *kom- "beside, near, by, with" (compare Old English ge-, German ge-). The prefix in Latin sometimes was used as an intensive.

Before vowels and aspirates, it is reduced to co-; before -g-, it is assimilated to cog- or con-; before -l-, assimilated to col-; before -r-, assimilated to cor-; before -c-, -d-, -j-, -n-, -q-, -s-, -t-, and -v-, it is assimilated to con-, which was so frequent that it often was used as the normal form.

1620s, "equality in value or circumstances," 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," a word of unknown and disputed origin. De Vaan is noncommittal. 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"). Meaning "a standard fixed by consent or by natural conditions, average or usual amount" is first attested 1767. Golf sense is attested by 1898, which led to the figurative use of par for the course for "fairly normal, what can be expected" (by 1928).