My latest (and extremely late) piece about the word nyorai is up at Néojaponisme.

Nyorai is the Japanese pronunciation of rúlái 如来, which is in turn the Chinese translation of tathāgata. The etymology of the original term is unclear, but in the context of the Mahayana Buddhism that swept China and later Japan, it refers to either the original Buddha or another who, like him, made a mercy mission to our world to spread truth and light. [...] Nyorai has also been applied to the Christian God. [...]

"The etymology of the original term is unclear": this is the part I wanted to expand on.

Wikipedia puts it thus:

Sanskrit grammar offers two possibilities for breaking up the compound: either Tathā and āgata or Tathā and gata. Tathā means "thus" in Sanskrit and Pali, and Buddhist thought takes this to refer to what is called "reality as-it-is" (Yathā-bhūta).

[...] Gata is the past passive participle of the verbal root gam (going, traveling). Āgata adds the verbal prefix Ā which gives the meaning "come, arrival, gone-unto." [...]

Thus in this interpretation Tathāgata means literally either, "The one who has gone to suchness" or, "The one who has arrived at suchness."

Note that the standard Chinese translation of the word, 如来, implicitly accepts the latter derivation (or a simpler version, like "[one who has] thus come"). The Iwanami Dictionary of Buddhism (岩波仏教辞典) confirms that almost all Chinese translations of Buddhist texts do indeed use 如来... which is actually kind of tantalizing — there are some texts that use 如去, "thus gone"? Hmm.