late 14c., ynneside "interior of the body," compound of in, adv. + side, n.. The adjective is 1610s, from the noun. Inside job "robbery, espionage, etc., committed by or with the help of a resident or servant of a place" is attested by 1887, American English (also, late 19c., early 20c., "indoors work"). Inside track "advantage" is metaphoric because those lanes are shorter on a curved track. Inside of, in reference to time, is from 1839.