The word lark has two unrelated meanings:

1. A small songbird with brown feathers, known for singing beautifully while flying high in the sky. Larks live in open fields and grasslands across Europe, Asia, and North America. The skylark is especially famous for its elaborate song, which inspired poets like Percy Bysshe Shelley, who wrote “To a Skylark” celebrating the bird's joyful music. Larks sing most actively at dawn, which is why people sometimes refer to early risers as being “up with the larks.”
2. A playful adventure or something done just for fun, without any serious purpose. When you go on a lark, you're doing something spontaneous and entertaining, like your family deciding on a whim to drive to the beach on a sunny Saturday. You might hear someone say, “We went to the fair just for a lark,” meaning they went purely for the fun of it, not because they needed anything there. If you do something “on a lark,” you're acting on a carefree impulse. A lark suggests lightheartedness and a spirit of fun: it's the opposite of a carefully planned, serious activity. The word as a verb means to play around or engage in harmless fun, as in larking about with friends after school.