double (n.)
mid-14c., "amount twice as great," also "duplicate copy," from double (adj.).
double (v.)
late 13c., "make double," from Old French dobler, from Latin duplare, from duplus "twofold, twich as much" from duo "two" (from PIE root *dwo- "two") + -plus "more" (see -plus). 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 (adj.)
early 13c., from Old French doble (10c.) "double, two-fold; two-faced, deceitful," from Latin duplus "twofold, twich as much" from duo "two" (from PIE root *dwo- "two") + -plus "more" (see -plus). Double standard in reference to morality and ethics is attested by 1871 but not common before mid-20c.; earlier it referred to monetary policy and bimetallism (1823). Military double time (1833) originally was 130 steps per minute.