translation - Ｗhat does っていうんだ mean?

I came across the following sentence:

僕が何を怖がるっていうんだ？

which was translated as "What am I supposed to be frightened of?"

I know that the first part means "what am I afraid of?"

I just can't see how adding that っていうんだ could change the meaning in that way.

I'm assuming that it's the same as というんだ but I still can't figure out how it affects the meaning of the sentence.

Could it be a mistranslation on the author's part?

Answer

僕が何を怖がるっていうんだ？

It's a rhetorical question (修辞疑問文/反語表現). It's not really a normal question that asks for an answer/reply. It means/implies "What would I be afraid of? -- No, I would be afraid of nothing!"

Examples:

• 誰が知っているというのか。
  "Who knows? (Nobody knows.)"


• どうすれば忘れられるというのか。
  "How could I forget? (I could never forget.)"


• それがどうしたっていうんだ。*
  "What would it matter? (It doesn't matter!)"


• やつらが金以外のなにを欲しがるというんだ？*　
  "What do they care for but money? (They only care for money!)"

_{The latter two examples are taken from Weblio例文辞典.}