lark (v.) Look up lark at Dictionary.com
"to play tricks, frolic," 1813; see lark (n.2). Related: Larked; larking.
lark (n.2) Look up lark at Dictionary.com
"spree, frolic, merry adventure," 1811, slang, of uncertain origin. Possibly a shortening of skylark (1809), sailors' slang for "play rough in the rigging of a ship" (larks were proverbial for high-flying). Or perhaps it is an alteration of English dialectal or colloquial lake/laik "to play, frolic, make sport" (c. 1300, from Old Norse leika "to play," from PIE *leig- (3) "to leap") with unetymological -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.
lark (n.1) Look up lark at Dictionary.com
songbird of the Old World, early 14c., earlier lauerche (c. 1200), from Old English lawerce (late Old English laferce), from Proto-Germanic *laiw(a)rikon (source also of Old Saxon lewerka, Frisian liurk, Old Norse lævirik, Dutch leeuwerik, German Lerche), a word of unknown origin.

Old English and Old Norse forms suggest a contracted compound, perhaps meaning "treason-worker," but "nothing is known in folklore to account for such a designation" [OED]. Noted for its early song and high flying (in contrast to its low nest). When the sky falls, we shall catch larks was an old proverb mocking foolish optimism. Latin alauda "the lark" (source of Italian aloda, Spanish alondra, Provençal alauza, French aloe, diminutive aloette) is said to be from Celtic.