Nancy Blackett is a fictional character in nine of the twelve juvenile novels in Arthur Ransome's Swallows and Amazons series of books. She is a bit of a tomboy—acting as captain of the dinghy, Amazon and usually directing her friends in their various adventures. Nancy apparently has no real-world counterpart as an inspirational source for Ransome but appears to be completely the author's creation. Nancy is sometimes critically viewed as a subversive character for girl readers. The character appeared in a 1963 BBC television adaptation of Swallows and Amazons as well as in a 1974 film adaptation of the book.
At the opening of the series in Swallows and Amazons, Nancy is 12 years old and lives with her younger sister Peggy and her widowed mother in a large Lakeland house called Beckfoot. In common with his treatment of many of his other characters, Ransome does not give a detailed description of Nancy, merely describing her as "bigger than John" (Walker). She is something of a tomboy who captains a dinghy called Amazon, usually wears a red pirate cap and often uses nautical or piratical words in her speech, such as "Jibbooms and bobstays" or the classic "shiver my timbers." In her first appearance in Swallows and Amazons, Peggy reveals that "Nancy" is itself a nickname derived from her affection for pirates: her real name is Ruth, which she changed to Nancy after her Uncle Jim pointed out that pirates are "ruthless." Nancy is the elder of the Amazons and older than the Swallows. She usually takes the lead in their adventures. Nancy has a lively imagination and usually thinks up adventures for her friends which she makes more exciting by imagining an exotic background such as climbing Kanchenjunga in Swallowdale instead of just a local mountain. Nancy is still a prime mover of the action even when she is prevented from taking direct part in the action such as when she is quarantined with mumps in Winter Holiday, or when she and Peggy are kept at home when the Great Aunt comes to stay.