double (adj.) Look up double at Dictionary.com
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.) Look up double at Dictionary.com
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.) Look up double at Dictionary.com
mid-14c., "amount twice as great," also "duplicate copy," from double (adj.).