On a lark is a phrase with unknown origins, though there are some theories. We will examine the definition of the phrase on a lark, where that term may have come from and some examples of its use in sentences.
On a lark describes something that is done on the spur of the moment, something that is done spontaneously and for fun. Something done on a lark has not been planned, but may be considered irresponsible in a fun way and childlike. The word lark in the term on a lark is used to mean frolicking or playing. The use of the word lark to mean frolicking first appeared in the early 1800s. It may stem from the word skylark, slang used by sailors to mean roughhousing in the rigging. It may date back to the Old Norse word leika which means to play. A third possibility is an Old English dialect word lake, which means to leap and play. In any case, the phrase on a lark conjures the image of a small bird flitting from tree to tree, and may be the reason the term has had staying power.
Almost on a lark, he envisioned a process whose elegance and simplicity would have delighted the architects he had idolized since he was a boy, passing through the streets of Dresden, days before the city was leveled by British and U.S. bombers in World War II. (The Minneapolis Star Tribune)
So when she noticed a listing for an available apartment inside, she went to the open house with a friend, on a lark. (Architectural Digest)
Poulin said he played a single game on a lark, allowing five goals against but avoiding a fight. (The Toronto Star)