Calling all Chinese linguists and etymological scholars!!
I have a hunch that the word 世界 (shi4 jie4, meaning “world” in modern Chinese) was coined by Kumarajiva (who headed the team that translated the Buddhist sutras into Chinese in the early 7th century) as a translation of the Sanskrit term lokadhatu (world). Elsewhere in the sutras, [i]lokah /i is rendered as 世, and [i]dhatu /i is rendered as 界 by Kumarajiva. My hunch is that the word 世界 started out as an esoteric Buddhist term but was subsequently adopted into the vernacular.
“The word 世界 in today’s Chinese language comes from the Buddhist sutras. It is not a modern word; its origins are traced back to Buddhism. 世 means time and 界 means space*; thus it covers the inseparability of time and space. This happens to be consistent with Einstein’s theory of relativity. The word used in ancient China to describe the chiliocosm [大千世界] was 天下, not 世界. A Buddhist term, 世界 is now widely used, but we should be aware of the source of the term. According to Volume 4 of the Surangama Sutra, 世 means flow and 界 means direction. That is, the flow of time comprises the triloka [three realms]–past, present and future–and space comprises the dasadisa [ten directions]–north, south, east, west, zenith, nadir, etc. In Buddhism the word 世界 refers not just to the Earth but to the entire universe, but in general parlance the word 世界 specifically refers to the Earth.”
The original meaning of 世 is “generation”, ultimately from 葉 “leaf”. Its meaning is extended to mean “world”. 界 actually means “boundary”. (Ref.: ABC Etymological Dictionary of Old Chinese)
I also found 界世. It’s not uncommon for disyllabic words to have a reverse order (同素異序)in the 佛典. Sometimes there are not any differences between the two varieties. However, most often the different varieties perform different syntactic functions. Bellow 界世性 is an adj. 'worldly '.