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nephew (n.)[nephew 词源字典]: c. 1300, from Old French neveu (Old North French nevu) "grandson, descendant," from Latin nepotem (nominative nepos) "sister's son, grandson, descendant," in post-Augustan Latin, "nephew," from PIE *nepot- "grandchild," and in a general sense, "male descendant other than son" (cognates: Sanskrit napat "grandson, descendant;" Old Persian napat- "grandson;" Old Lithuanian nepuotis "grandson;" Dutch neef; German Neffe "nephew;" Old Irish nia, genitive niath "son of a sister," Welsh nei). Used in English in all the classical senses until meaning narrowed in 17c., and also as a euphemism for "the illegitimate son of an ecclesiastic" (1580s). The Old English cognate, nefa "nephew, stepson, grandson, second cousin" survived to 16c.[nephew etymology, nephew origin, 英语词源]
step-: Old English steop-, with connotations of "loss," in combinations like steopcild "orphan," related to astiepan, bestiepan "to bereave, to deprive of parents or children," from Proto-Germanic *steupa- "bereft" (cognates: Old Frisian stiap-, Old Norse stjup-, Swedish styv-, Middle Low German stef-, Dutch stief-, Old High German stiof-, German stief-), literally "pushed out," from PIE *steup-, from root *(s)teu- (1) "to push, stick, knock," with derivatives referring to fragments (see steep (adj.)). Barnhart suggests the forms in -f- are by assimilation of the first sound in following words for "father."

Etymologically, a stepfather or stepmother is one who becomes father or mother to an orphan, but the notion of orphanage faded in 20c. and came to denote simply relation through marriage. For sense evolution, compare Latin privignus "stepson," related to privus "deprived." Compare orphan (n.).
step-son (n.): also stepson, Old English steopsunu; see step- + son.