The story
From the thought experiment conducted by Albert Einstein, in which one of a pair of twins sets off in a spaceship on a trip at high speed, and due to the effects of relativity, his effective clock ticks slower than the twin who remains at home, not encountering relativistic effects, so that when the spacetraveller twin returns home to compare clocks, the spacetraveller will have experienced less elapsed time than the homebody, thus having aged less than the homebody twin and being younger.
Etymology from Wiktionary, CC BY-SA 4.0.