The OED definition of larky, adj.: “Inclined or ready for a lark; frolicsome, sportive.” The verb form is commonly used in the gerund and followed by about. As Julie Andrews’ Mary Poppins famously instructed Dick Van Dyke’s Bert in the Disney film (words and music by Richard M. and Robert B. Sherman): “None of your larking about!”
It is a portrait of the larky upper-class sleuth of King’s College, Cambridge, and his thunderous middle-class quarry at the red-brick University of Newcastle. (Richard Eder, New York Times, August 13, 2000)/…Robert Wagner, who enjoyed his biggest success on Hart To Hart and It Takes A Thief, this larky series about an urbane jewel thief recruited to steal for the government when they’re not free to act. (Huffington Post, November 17, 2011)
notoneoffbritishisms.com 
BrE – Lark as a noun or a verb, never an adjective that I have heard/seen.
OED has a lot of cites, starting with Punch in 1841: “The old girl has her two nieces home for the holidays—devilish handsome, larky girls.” More recently, Richard Eyre writes of John Neville in yesterday’s Guardian: “John was infectiously anarchic – wild, larky and raffish as well as supremely skilful as an actor and inspiring as the leader of a company.”
I know it, I’ve read it, it’s perfectly fine a a synonym, but I’ve never heard it in American speech.
But what did he mean by “thunderous middle-class quarry”? Thunderingly middle-class? That would mean “very”, but “thunderous” means “about to explode with rage”.
I just came on the phrase “some larkish slumming” at the bottom right of an essay on alternative history by Thoimas Mallon on page 88 of the November 21 issue of The New Yorker. I still think it’s over the top.
Marc, thanks. I neglected to mention “larkish”–according to the OED, it first appears in 1823, thus predating “larky.” Here’s an 1882 quote from The Echo: “Foote lost his leg owing to amputation caused by a larkish exploit with the Duke of York.”
I doubt this’ll migrate anytime soon, but there’s a similar slang word from Yorkshire, which is actually derived from Viking. Lekkin(g) or laking [the latter I’ve never heard used, but apparently it exists] means playing.
Laiking does exist: it means to play, and must be cognate with larking. I live in Leeds, in Yorkshire, and laiking is still used by older people, though as a conscious use of the dialect they had when much younger. “Are we laiking?” is used for “Are we going out?”, always in the context of going out to drink beer.
In east Leeds, skipping school was always “legging off”, which I take to be a variation.
The skylark, of course, is a bird who, flying high and singing with joy, is definitely “up for a lark.”