Of uncertain origin. From the 18th century it is clearly used as a pet form of Ann (see Nan), but it may originally have been a similar formation deriving from the common medieval given name Annis, a vernacular form of Agnes. Nowadays it is an independent name, and was especially popular in America in the 1930s, 40s, and 50s. A meaning of the name Nancy is Grace.
Scottish and English: nickname for a large (tall or fat) person, from Middle English muche ‘great’, a shortened form (probably a back-formation, as if from a diminutive with the Anglo-Norman French suffix -el) of muchel, Old English mycel ‘great’, ‘large’.