Etymology
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fold (v.)

Old English faldan (Mercian), fealdan (West Saxon), transitive, "to bend (cloth) back over itself, wrap up, furl," class VII strong verb (past tense feold, past participle fealden), from Proto-Germanic *falthan, *faldan (source also of Middle Dutch vouden, Dutch vouwen, Old Norse falda, Middle Low German volden, Old High German faldan, German falten, Gothic falþan), from PIE *pol-to-, suffixed form of root *pel- (2) "to fold."

Of the arms, from late Old English. Intransitive sense "become doubled upon itself" is from c. 1300 (of the body); earlier "give way, fail" (mid-13c.). Sense of "to yield to pressure" is from late 14c. The weak conjugation developed from 15c. Related: Folded; folding.

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fold (n.1)
"pen or enclosure for sheep or other domestic animals," Old English falæd, falud "stall, stable, cattle-pen," a general Germanic word (cognates: East Frisian folt "enclosure, dunghill," Dutch vaalt "dunghill," Danish fold "pen for sheep"), of uncertain origin. Figurative use by mid-14c.
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fold (n.2)
"a bend or ply in anything," mid-13c., from fold (v.). Compare similarly formed Middle Dutch voude, Dutch vouw, Old High German falt, German Falte, Old Norse faldr.
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fold-out (n.)
larger page, inserted folded, in a book, magazine, etc., 1961, from fold (v.) + out (adv.).
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-fold 

multiplicative word-forming element attached to numerals, from Old English -feald, Northumbrian -fald, from Proto-Germanic *-falda- (cognates: Old Saxon -fald, Old Frisian -fald, Old Norse -faldr, Dutch -voud, German -falt, Gothic falþs), combining form of *falthan, from PIE *polt-, extended form of root *pel- (2) "to fold."

The same root yielded fold (v.) and perhaps also Greek -ploid, -plos and Latin -plus (see -plus). Native words with it have been crowded out by Latinate double, triple, etc., but it persists in manifold, hundredfold, etc.

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bifold (adj.)
"double, of two kinds," c. 1600; see bi- "two" + -fold.
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sixtyfold 
also sixty-fold, Old English sixtigfeald; see sixty + -fold.
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fivefold (adv.)
1570s, from earlier use as an adjective, from Old English fiffeald (adj.); see five + -fold.
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tenfold (adj.)
Old English tienfeald; see ten + -fold. As an adverb in modern use from 1530s.
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