lark (n.1)
"songbird," early 14c., earlier lauerche (c.1200), from Old English lawerce (late Old English laferce), from Proto-Germanic *laiw(a)rikon (cognates: Old Saxon lewerka, Frisian liurk, Old Norse lævirik, Dutch leeuwerik, German Lerche), of unknown origin. Some Old English and Old Norse forms suggest a compound meaning "treason-worker," but there is no folk tale to explain or support this.
lark (n.2)
"spree, frolic," 1811, possibly shortening of skylark (1809), sailors' slang "play rough in the rigging of a ship" (larks were proverbial for high-flying), or from English dialectal lake/laik "to play" (c.1300, from Old Norse leika "to play," from PIE *leig- "to leap") with intrusive -r- common in southern British dialect. The verb lake, considered characteristic of Northern English vocabulary, is the opposite of work but lacks the other meanings of play. As a verb, from 1813. Related: Larked; larking.