Lehrjahre (n.)

Old English leornian "to get knowledge, be cultivated; study, read, think about," from Proto-Germanic *lisnojanan (cognates: Old Frisian lernia, Middle Dutch leeren, Dutch leren, Old High German lernen, German lernen "to learn," Gothic lais "I know"), with a base sense of "to follow or find the track," from PIE root *lois- "furrow, track." It is related to German Gleis "track," and to Old English læst "sole of the foot" (see last (n.1)).

From c. 1200 as "to hear of, ascertain." Transitive use (He learned me (how) to read), now considered vulgar (except in reflexive expressions, I learn English), was acceptable from c. 1200 until early 19c. It is preserved in past-participle adjective learned "having knowledge gained by study." Old English also had læran "to teach" (see lere). Related: Learning.

Old English gear (West Saxon), ger (Anglian) "year," from Proto-Germanic *jēr "year" (source also of Old Saxon, Old High German jar, Old Norse ar, Danish aar, Old Frisian ger, Dutch jaar, German Jahr, Gothic jer "year"), from PIE *yer-o-, from root *yer- "year, season" (source also of Avestan yare (nominative singular) "year;" Greek hōra "year, season, any part of a year," also "any part of a day, hour;" Old Church Slavonic jaru, Bohemian jaro "spring;" Latin hornus "of this year;" Old Persian dušiyaram "famine," literally "bad year"). Probably [Watkins] originally "that which makes [a complete cycle]," and from verbal root meaning "to do, make."

Proto-Indo-European root meaning "furrow, track." 

It forms all or part of: delirious; delirium; last (n.1) "wooden model of a human foot used by shoemakers;" last (v.) "endure, go on existing;" learn; learning; Lehrjahre; lore.

It is the hypothetical source of/evidence for its existence is provided by: Latin lira "furrow;" Old Prussian lyso "field bed;" Old Church Slavonic lexa "field bed, furrow;" Old High German leisa "track," Gothic laistjan "to follow," Old English læran "to teach."