Etymology
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file (n.1)

1520s, "string or wire on which documents are strung," from French file "a row" (15c.), noun derived from filer "string documents; spin thread" (see file (v.1)). The literal sense explains why from the beginning until recently things were generally on file (or upon file). The meaning "collection of papers systematically arranged for ready reference" is from 1620s; computer sense is from 1954. The sense "row of persons or things one behind another" (1590s) is originally military, from the French verb in the sense of "march in file." Meaning "line of squares on a chessboard running directly from player to player" is from 1610s.

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file (v.2)
"to smooth or abrade with a file," early 13c., from Old English filian, from the source of file (n.2). Related: Filed; filing.
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file (v.1)

"place (papers) in consecutive order for future reference," mid-15c., from Old French filer "string documents on a thread or wire for preservation or reference" (15c.), earlier "to spin thread," from fil "thread, string" (12c.), from Latin filum "a thread, string; thread of fate; cord, filament," from PIE *gwhis-lom, suffixed form of root *gwhi- "thread, tendon." The notion is of documents hung up on a line in consecutive order for ease of reference.

File (filacium) is a threed or wyer, whereon writs, or other exhibits in courts, are fastened for the better keeping of them. [Cowel, "The Interpreter," 1607]

Methods have become more sophisticated, but the word has stuck. Meaning "place among the records of a court or office" is from 1510s; of newspaper reporters sending in stories, 1954. Intransitive sense "march in a line (as soldiers do) one after another" is from 1610s. Related: Filed; filing.

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file (n.2)

metal tool for abrading or smoothing, Old English feol (Mercian fil) "file," from Proto-Germanic *fihalo "cutting tool" (source also of Old Saxon fila, Old High German fila, Middle Dutch vile, Dutch vijl, German Feile), probably from PIE root *peig- "to cut, mark by incision" (source also of Old Church Slavonic pila "file, saw," Lithuanian piela "file"). Century Dictionary (1906) lists 60 named varieties of them. Nail file (for the fingernails) is by 1819.

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nail-file (n.)

"small, flat, single-cut file for trimming the fingernails," by 1823, from nail (n.) + file (n.2).

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rank and file (n.)

1590s, in reference to the horizontal and vertical lines of soldiers marching in formation, from rank (n.) in the military sense of "number of soldiers drawn up in a line abreast" (1570s) + file (n.1). Thence generalized to "common soldiers" (1796) and "common people, general body" of any group (1860).

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filing (n.)
1712, verbal noun from file (v.1). Filing cabinet is from 1883.
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fillet (n.)
early 14c., "little headband," from Old French filet "thread, filament; strip, ligament" (12c.), diminutive of fil "thread" (see file (v.1)). Sense of "cut of meat or fish" is from late 14c., apparently so called because it was prepared by being tied up with a string.
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*gwhi- 
*gwhī-, Proto-Indo-European root meaning "thread, tendon."

It forms all or part of: defile (n.) "narrow passage;" enfilade; filament; file (v.1) "place (papers) in consecutive order for future reference;" filigree; filipendulous; fillet; profile.

It is the hypothetical source of/evidence for its existence is provided by: Avestan jya- "bowstring;" Latin filum "a thread, string;" Armenian jil "sinew, string, line;" Lithuanian gysla "vein, sinew;" Old Church Slavonic zila "vein."
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enfilade (n.)
1706, a string of things in a straight line, from French enfilade, from Old French enfiler (13c.) "to thread (a needle) on a string; pierce from end to end," from en- "put on" (see en- (1)) + fil "thread" (see file (v.1)). Used of rows of apartments and lines of trees before military sense came to predominate: "a firing with a straight passage down ranks of men, channels in fortifications, etc." (1796). As a verb from 1706 in the military sense, "rake with shot through the full length." Related: Enfiladed; enfilading. The Old French verb was borrowed in Middle English as enfile "to put (something) on a thread or string."
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