1. To frolic; sport
1. A brown-speckled European lark, Alauda arvensis, famed for its melodious song
|A skylark (source)|
Later skylark became nautical slang for "wanton play about the rigging and tops," which lead to the verb definition by at least 1809.
Today's word and the first definition were both taken from Dictionary.com's 'Word of the Day' for Sunday, July 10
Etymologies come from the Oxford English Dictionary and/or Etymonline.com