Chippie Sauce is a condiment used in Edinburgh, Scotland on chips (aka "French Fries" in North America.) A rival to the traditional salt and vinegar, it is still pretty much unique to Edinburgh and is held by fans to be a defining cultural mark of the city. A "chippie" or "chippy" is a shop that cooks and sells chips.
Chippie Sauce is a brown sauce, but far thinner than the "brown sauces" such as HP and Daddy's that appear on tables. People will put both salt and the sauce on their chips, referred to as "salt'n'soss" or "saucy chips."
The sauce is not sold in stores -- though some chippies will sell you a bottle to take home with your order.
Consequently, there is much speculation as to what the recipe is. Opinion does vary, but those people who profess to own or work in a chip shop  say that it is simply two-thirds brown sauce to one third plain, cold tap water, shaken. The brown sauce should be a cheaper one, such as Gold Star brand (HP is too fancy for this.) If you can't get Gold Star, get the cheapest you can find from a cash and carry.
The same sources say that it is a popular misconception amongst the public that white (aka spirit) vinegar is what is used to dilute the brown sauce. All in all, with the cheaper brown sauce, and just plain tap water, they say, it is a thoroughly thrifty Scots concoction.
The sauce is usually put into refillable plastic squeezy bottles so that it can just be squeezed out onto a customer's order for them.