The word “fushigi” in Japanese, written in Kanji as 不思議, has several related meanings including “strange”, “miraculous”, and “odd”. It can be used as both a noun or a na-adjective. I always like to look at words’ definitions in Japanese to try and get closer to the root meaning. Here is the word’s first definition in Dictionary Goo, with my rough translation:
“Something which has a reason that cannot be imagined or conceived of under normal circumstances. Something which cannot be explained. Alternatively, such a state.「A mysterious event」「Success is not surprising」”
Fushigi can also be translated as “wonder”, and the “seven wonders of the world” is said in Japanese with the phrase “世界の七不思議” (sekai no nanafushigi). Note that this isn’t the normal way of counting 7 which would be more like 七つ (nanatsu).
In some cases you might see the な omitted when used as an adjective, like in the title of the 1990s Manga “fushigi yuugi” (ふしぎ遊戯). The word 遊戯 here means some type of “game” or “play” (not a broadway “play”, but in a general sense) and is actually the same word from the Anime series and card game “Yu-Gi-Oh!”, which is written 遊戯王 (The last character means ‘king’).
摩訶不思議 (maka fushigi) is a related word that has a stronger meaning, along the lines of “a profound mystery” (非常に不思議）
不思議 can also be used as an adverb by adding a に, and is often used in the phrase ”不思議に思う”。
- There is something I thought was strange. “literally: There is something I thought strangely about”
You can also use the pattern “te form of a verb + も + 不思議じゃない” to mean something is probable to happen.
- I’d not be surprised if he won (the contest).
The words 謎 (nazo) and 神秘的 (shinpiteki) are rough synonyms of 不思議。 謎 is a noun and has a nuance of “a puzzle”, and 神秘的 is a na-adjective whose meaning is closer to “mystical”.