Perennial means lasting or returning year after year. It comes back whether you want it to or not.
Gardening has the precise sense, since a perennial plant lives for several years and comes up again each spring, in contrast to an annual, which does everything in one season and dies. That distinction is the first thing anybody learns about planting a border. The figurative sense is about recurring things, so a perennial problem and a perennial favourite both return every year. Perennially is the adverb.
A perennial lasts through the years. Latin perennis is per, through, plus annus, a year, so the plant that returns and the perennial complaint are both measured in the same unit.
Proto-Indo-European
*per-
Proto-Indo-European
*h₂et- — to go
Latin
perennis — lasting through the whole year or for several years, perennial; continual, everlasting, perpetual
English, today
perennial
Built from
The adjective is borrowed from Latin perennis (“lasting through the whole year or for several years, perennial; continual, everlasting, perpetual”) + -al. Perennis is derived from per- (“completive or intensifying prefix with the sense of doing something all the way through or entirely”) + annus (“year; season, time”) (possibly ultimately from Proto-Indo-European h₂et- (“to go”)). by surface analysis, per- + -ennial. The noun is derived from the adjective. 2
Etymology from Wiktionary, CC BY-SA