‘Knock down ginger’ is the age old practice of knocking on someone’s front door and running away in a mischievous fashion; something I still enjoy playing to this day when the mood strikes me.
In this increasingly technological age, the practice can also include the ringing of doorbells or flat-entry intercom systems; personally, I still prefer knocking on the door with my knuckles because it makes you sound like a baliff or the old bill! “Open up you naughty slags, it’s the fuzz!”.
‘Knock down ginger’ has absolutely nothing to do with beating up ginger people, nor does it have anything to do with the Weasley family in Harry Potter. ‘Knock, Knock, Ginger’ or ‘Knocky Door Ginger’ were the original names for the game which dates back to the 19th century in England; possibly might even be a Cornish traditional holiday of Nickanan Night according to the intersnot.
Google was a helpful whore as always and gave up her goodies about the origins of the prank on the first date, apparently, the saying ‘Knock down ginger’ comes from an English rhyme:
Ginger, Ginger broke a winder
Hit the winda – Crack!
The baker came out to give ‘im a clout.
And landed on his back
So as you see, I have nothing against Ginger’s whatsoever!