To "hit for six", alternatively "knocked for six", is a British idiom meaning to hit something, or be hit by something, very hard. It can be used in reference to a physical blow, but it is often used metaphorically:

I was so shocked by the news, it knocked me for six.

The saying originates from the sport of cricket, where it means to hit the ball so hard it goes out of the boundary.

"The hun", as you correctly found, was a British term for the German army. It originated during World War 1, although it may well have continued to be used during the second, too.

To "hit for six", alternatively "knocked for six", is a British idiom meaning to hit something, or be hit by something, very hard. It can be used in reference to a physical blow, but it is often used metaphorically:

I was so shocked by the news, it knocked me for six.

The saying originates from the sport of cricket, where it means to hit the ball so hard it goes out of the boundary.

"The hun", as you correctly found, was a British term for the German army. It originated during World War 1, although it may well have continued to be used during the second, too. It was a reference to the nomadic tribe from history, who were not from Germany, but considered to be barbarians.