"through, by means of," 1580s (earlier in various Latin and French phrases, in the latter often par), from Latin per "through, during, by means of, on account of, as in," from PIE root *per- (1) "forward," hence "through, in front of, before, first, chief, toward, near, around, against."
late 14c., "appointed by the year;" c. 1400, ""occurring or done once a year," from Old French annuel "yearly" (12c.) or directly from Medieval Latin annualis "yearly," corresponding to Latin annalis as adjective form of annus "year."
This is reconstructed to be from Proto-Italic *atno- "year" (compare Oscan akno- "year, holiday, time of offering"), from PIE *at-no- "which goes," also "a year" (as "going around"), suffixed form of root *at- "to go" (source also of Sanskrit atati "goes, wanders," atamana- "to travel, wander," atya- "steed, runner"). The root also has Germanic derivatives meaning "a year," such as Gothic aþnam (dative plural) "year."
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Definitions of per annum from WordNet
per annum (adv.)
by the year; every year (usually with reference to a sum of money paid or received);