"You can say that again!"
Translation:Das kannst du laut sagen!
No, both the English and German phrases are idioms with essentially the same meaning.
What is the meaning of this idiom? I am neither German nor English native speaker. I don't see how "laut" can be used for "again" here.
The literal meaning is something like what jdgomezm said. It's normally used to express strong approval to something someone said. 'Heute ist es wieder sehr kalt. - Das kannst du laut sagen!'