世 most likely comes from three 十 (ten) joined — three tens make thirty, and reckoning one 世代 (generation) at about thirty years gave "generation, an age, the world, an era." The span of a generation passing is the world itself.

Korean hanja-sound "se": 世界 (segye, world), 世上 (sesang, the world), 世代 (sedae, generation), 世紀 (segi, century), 出世 (chulse, success in life), 別世 (byeolse, passing away), 亂世 (nanse, troubled times). With 代 seen earlier it forms 世代 (generation).

Mandarin: shì, 4th tone. 世界 (shìjiè, world), 世纪 (shìjì, century), 世代 (shìdài, generation), 去世 (qùshì, to pass away), 世故 (shìgù, worldly-wise). 世界 (shìjiè, world) is one of the most frequent everyday Mandarin words.

Japanese: on-readings セイ (sei) and セ (se), kun よ (yo). 世界 (sekai, world — a セ reading), 世紀 (seiki, century), 世間 (seken, society / the world), 出世 (shusse, success in life), 世の中 (yo no naka, the world). Japanese says 世の中 (yo no naka) and 世間 (seken) for "the world."

Memory aid: three tens (十) joined into thirty years, one generation — generations passing become the world.