What is the X in X-mas?
c.1300, from Old French niece "niece, granddaughter" (12c., Modern French nièce), earlier niepce, from Latin neptia (also source of Portuguese neta, Spanish nieta), from neptis "granddaughter," in Late Latin "niece," fem. of nepos "grandson, nephew" (see nephew). Replaced Old English nift, from Proto-Germanic *neftiz, from the same PIE root (Old English also used broðordohter and nefene).
Until c.1600, it also commonly meant "a granddaughter" or any remote female descendant. Cf. cognate Spanish nieta, Old Lithuanian nepte, Sanskrit naptih "granddaughter;" Czech net, Old Irish necht, Welsh nith, German Nichte "niece."