「ex-boyfriend」的 「ex」英文全名是甚麼 ？

Ermmm... I can't really understand your question, but *EX* is a preffix which means *something that was but now it's not*

Ex-boyfriend = Someone's boyfriend who now it's not her boyfriend.
Ex-president = Someone who were a President in the past, but now she/he's not.
Ex-superstar = Someone who isn't a Superstar in the present.
...and so on.

根據網上資料, http://en.wikipedia.org/wiki/Ex-
ex 是古希臘字 εκ-, εχ-, εχω- 的近似字.

ex既全寫都係ex
放係一個字之前去形容前度的意思

ex- /eks/ prefix (in nouns) former: ex-wife ex-president