Real Dictionary

lark
You use lark in expressions such as this acting lark and the writing lark to indicate humorously that you think an activity or job is amusing, foolish, or unnecessary.
countable noun: A lark is a small brown bird which makes a pleasant sound.
countable noun: If you say that doing something is a lark, you mean that it is fun, although perhaps naughty or dangerous.

lark in American English 1
a frolic or spree
to play or frolic; have a merry time
noun: any of numerous, chiefly Old World oscine birds, of the family Alaudidae, characterized by an unusually long, straight hind claw, esp. the skylark Alauda arvensis

lark in American English 2
any of a large family (Alaudidae) of chiefly Old World passerine birds, including the skylark and horned lark
intransitive verb: to have fun; frolic; romp
noun: a merry, carefree adventure; frolic; escapade

lark in British English 1
noun: any brown songbird of the predominantly Old World family Alaudidae, esp the skylark: noted for their singing

lark in British English 2
noun: a carefree adventure or frolic