Meaning

noun a junior Girl Scout

noun (folklore) fairies that are somewhat mischievous

noun square or bar of very rich chocolate cake usually with nuts

originFrom brownie (“a helpful elf”). Specifically, inspired by the story "The Brownies" by Juliana Horatia Ewing (published in 1870, The Brownies and Other Tales), in which two children, Tommy and Betty, learn that children can be helpful brownies or lazy boggarts. But compare with Dutch kabouter (“a very young female member of the scout movement”) (literally, a "gnome").

Etymology from Wiktionary, CC BY-SA 3.0