What does でしかない mean in this sentence?

Question

As far as I know, でしかない means: no more than or merely, but in this sentence that meaning doesn't seem to fit.

• それを専攻をするのは天才でしかないよ！

So, what exactly does this expression mean in this sentence?

Answer 1

Assuming it was written by a native speaker, I would call this a humorous sentence rather than a perfectly standard one. The sentence literally does mean:

それを専攻するのは天才でしかないよ！
Majoring in that means [someone] is merely a genius!

But it effectively means:

Majoring in that means [someone] is nothing but a genius!

しかない is normally used in a negative way ("only", "merely", "no more than", etc), but here it is used to refer to something desirable, which is why you were puzzled, right? In reality, people may say something like this half-jokingly.

• 「自信ありますか？」「自信しかないです！」
  "Are you confident?" "I have nothing except for confidence!"
• 幸せでしかない。
  I'm nothing but happy.

Don't try them at home unless you're really fluent in Japanese.

Answer 2

A translation of your Japanese sentence would be:

"Man, only a genius would choose that as their major!"

If this sentence was directed at you personally, then:

"Man, you must be a genius for having chosen that major!"