This is the one the kids always get wrong in the end of year exams. As I have no idea where the derivative comes from I looked this up, no one else seems to know either! One source gives its earliest reference as being in 1979, I know it was in use before then.
Germans = German bands = hands.
What 'German bands' refers to is anyone's guess, rhyming slang tends not to be something completely randomly made up, so it probably refers to something that was in more common use. The internet seems to want to show me pictures of Kraftwerk, so no help at all.
I'm open to suggestions for the etymology...