A small songbird known for its beautiful song; or a fun, playful adventure or prank.
From Old English 'laverock,' related to Old Norse 'loerkr,' from Proto-Germanic 'lairik.' The bird sense is old; the 'fun adventure' sense developed in the 18th century.
The lark became synonymous with fun and adventure because of its joyful song and the way it spirals upward in flight—medieval poets used larks as symbols of pure joy and freedom. So when people started saying 'what a lark!' they were imagining the bird's carefree spirit.
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