annual (adj.)
late 14c., from Old French annuel (12c.) or directly from Late Latin annualem (nominative annualis), corresponding to Latin annalis as adjective form of annus "year," from PIE *at-no-, from root *at- "to go," on notion of "period gone through" (cognates: Sanskrit atati "goes, wanders," Gothic aþnam (dative plural) "year," Oscan akno- "year, holiday, time of offering"). Used of plants since 1710.
annual (n.)
c.1400, originally "service commemorating the anniversary of a person's death," from annual (adj.). By 1824 as short for annual plant.