The Collaborative International Dictionary of English v.0.48:

Lark \Lark\ (l[aum]rk), n. [Perh fr. AS. l[=a]c play, sport. Cf.
   Lake, v. i.]
   A frolic; a jolly time. [Colloq.] --Dickens.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lark \Lark\, v. i. [imp. & p. p. Larked (l[aum]rkt); p. pr. &
   vb. n. Larking.]
   To sport; to frolic. [Colloq.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lark \Lark\, n. [OE. larke, laverock, AS. l[=a]werce; akin to D.
   leeuwerik, LG. lewerke, OHG. l[=e]rahha, G. lerche, Sw.
   l[aum]rka, Dan. lerke, Icel. l[ae]virki.] (Zool.)
   Any one numerous species of singing birds of the genus
   Alauda and allied genera (family Alaudid[ae]). They
   mostly belong to Europe, Asia, and Northern Africa. In
   America they are represented by the shore larks, or horned
   larks, of the genus Otocoris. The true larks have
   holaspidean tarsi, very long hind claws, and, usually, dull,
   sandy brown colors.
   [1913 Webster]

   Note: The European skylark, or lark of the poets (Alauda
         arvensis), is of a brown mottled color, and is noted
         for its clear and sweet song, uttered as it rises and
         descends almost perpendicularly in the air. It is
         considered a table delicacy, and immense numbers are
         killed for the markets. Other well-known European
         species are the crested, or tufted, lark (Alauda
         cristata), and the wood lark (Alauda arborea). The
         pipits, or titlarks, of the genus Anthus (family
         Motacillid[ae]) are often called larks. See Pipit.
         The American meadow larks, of the genus Sturnella,
         are allied to the starlings. See Meadow Lark. The
         Australian bush lark is Mirafra Horsfieldii. See
         Shore lark.
         [1913 Webster]

   Lark bunting (Zool.), a fringilline bird (Calamospiza
      melanocorys) found on the plains of the Western United
      States.

   Lark sparrow (Zool.), a sparrow (Chondestes grammacus),
      found in the Mississippi Valley and the Western United
      States.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lark \Lark\, v. i.
   To catch larks; as, to go larking.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lark
    n 1: North American songbirds having a yellow breast [syn:
         meadowlark, lark]
    2: a songbird that lives mainly on the ground in open country;
       has streaky brown plumage [syn: pipit, titlark, lark]
    3: any of numerous predominantly Old World birds noted for their
       singing
    4: any carefree episode [syn: escapade, lark]
    v 1: play boisterously; "The children frolicked in the garden";
         "the gamboling lambs in the meadows"; "The toddlers romped
         in the playroom" [syn: frolic, lark, rollick,
         skylark, disport, sport, cavort, gambol, frisk,
         romp, run around, lark about]

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "lark":
   Philomel, adventure, antic, bat, bender, binge, bout, bulbul, bust,
   canary, caper, carousal, carouse, cavort, celebrate, celebration,
   cuckoo, cut loose, debauch, drinking bout, eagle, escapade,
   feathered songster, fling, frolic, gambol, game, hell around,
   horseplay, jape, jollify, jolly, let go, let loose, let off steam,
   make merry, make whoopee, mavis, mischief, mockingbird,
   monkeyshine, nightingale, orgy, oriole, play, ploy, practical joke,
   prank, raise hell, randan, randy, revel, ringdove, rocket, roister,
   rollick, romp, see life, shenanigan, shenanigans, shines,
   singing bird, skylark, skyrocket, song sparrow, songbird, songster,
   sport, spree, step out, tear, thrush, tomfoolery, toot, trick,
   wanton, warbler, whoop it up, wingding