İts slang. You can signify how surprised you are about sth. For example: Günter ist den Marathon unten 3 Stunden gelaufen. Alter falter!

It is slang. Alter Falter = Wow

If there is a rhyme it's often slang and can't be translated literally. For instance: Aus die Maus. -> Stop what you are doing. Alles fit im Schritt? -> How are you?/Are you fine?