foldyoudaoicibaDictYouDict[fold 词源字典]
fold: [OE] The verb fold comes ultimately from the Indo-European base *pel-, which also produced Latin plicāre ‘fold’ (source of or related to English accomplice, complicated, explicit, perplex, plait, pleat, pliant, pliers, plight, ply, reply, and supple) and the final element -ple or -ble in such words as simple, double, or triple (which are hence related to the parallel Germanic formations twofold, threefold, etc).

Its Germanic descendant was *falthan, from which are descended German falten, Dutch vouwen, Danish folde, and English fold. The noun fold ‘enclosure for animals’ is of Germanic origin (Dutch has the related vaalt), but its distant antecedents are unknown.
=> accomplice, complicated, explicit, perplex, plait, pleat, pliers, plight, ply, reply, supple[fold etymology, fold origin, 英语词源]
fold (v.)youdaoicibaDictYouDict
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 (cognates: Middle Dutch vouden, Dutch vouwen, Old Norse falda, Middle Low German volden, Old High German faldan, German falten, Gothic falþan).

The Germanic words are from PIE *pel-to- (cognates: Sanskrit putah "fold, pocket," Albanian pale "fold," Middle Irish alt "a joint," Lithuanian pleta "I plait"), from root *pel- (3) "to fold" (also source of Greek ploos "fold," Latin -plus).

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.
fold (n.1)youdaoicibaDictYouDict
"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.
fold (n.2)youdaoicibaDictYouDict
"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.