doubleyoudaoicibaDictYouDict[double 词源字典]
double: [13] Double comes via Old French doble or duble from Latin duplus (direct source of English duple [16]). This was a compound adjective formed from duo ‘two’ and an Indo- European element *pl- which denoted ‘folding’ (it is present also in English fold and ply). The same semantic elements went to make up English twofold, and indeed duplex (see DUPLICATE), and also Greek diplous (source of English diploma and diplomat).

The underlying meaning of doublet ‘close-fitting jacket’ [14] (borrowed from French doublet, a derivative of double) is ‘something folded’, while doubloon [17], borrowed via French doublon from Spanish doblón (a derivative of doble ‘double’) was originally a gold coin worth ‘double’ a pistole.
=> diploma, diplomat, dub, duplicate, fold, ply[double etymology, double origin, 英语词源]
double (adj.)youdaoicibaDictYouDict
early 13c., from Old French doble (10c.) "double, two-fold; two-faced, deceitful," from Latin duplus "twofold, twich as much" from duo "two" (see two) + -plus "more" (see -plus). Double standard attested by 1951. Military double time (1833) originally was 130 steps per minute.
double (v.)youdaoicibaDictYouDict
late 13c., "make double," from Old French dobler, from Latin duplare, from duplus (see double (adj.)). Meaning "to work as, in addition to one's regular job" is c. 1920, circus slang, from performers who also played in the band. Related: Doubled; doubling. To double up bodily is from 1814.
A blow on the stomach "doubles up" the boxer, and occasions that gasping and crowing which sufficiently indicate the cause of the injury .... [Donald Walker, "Defensive Exercises," 1840]
double (n.)youdaoicibaDictYouDict
mid-14c., "amount twice as great," also "duplicate copy," from double (adj.).