Sprightly comes from spright, an archaic version of the word we now use for an elf or fairy: sprite. Ariel from William Shakespeare's The Tempest and the leprechaun of Irish mythology are often referred to as sprites, and it's no coincidence that both are characterized by their light, flitting movements and mannerisms. Sprite derives via Middle English and Old French from the Latin spiritus, which of course gives us spirit as well. A similar-looking adjective that can describe someone who is nimble and energetic is spry, but the origin of that word is not known.