Haggis is a Scottish dish that is like a large sausage, or a large steamed pudding, with a sheep’s stomach used as the wrapper for the stuffing inside.
In some ways, it is an original version of a steamed pudding. Seamed puddings were first wrapped in intestines, then from the 1600s in cloth, then from the Victorian era put in pudding dishes.
A Haggis is stuffed with minced offal, bound together with oatmeal, beef suet and a liquid, and flavoured with onions and spices. Oatmeal is about 90% of the mixture.
The combination of offal used varies by recipe (heart, liver, lungs, etc.) The animal it is from can be beef, lamb, or game. The liquid can be stock, gravy, red wine, etc. Using beef suet instead of mutton fat is a modern practice.
This mixture is then stuffed into a sheep’s stomach. The stomach used to be sealed with wooden skewers; now it is sewn up with a needle and thread. The stuffed stomach is then simmered for about three hours.
The stomach linings can occasionally burst during the simmering if too much water is absorbed by the filling, causing it to swell. To help avoid this, it can be pricked with a needle during cooking to help relieve internal pressure.
It is served in slices, usually as a main course, with potatoes and mashed turnips. It is served particularly on Robbie Burns Day (25th January) and St Andrew’s Day (30th November.) Some restaurants in Scotland now serve small slices as an appetizer.
You can buy them ready to go at butchers now. Modern ones might use plastic bags instead as a wrapper.
Vegetarian and canned versions are also available now.
In fish and chip shops in Scotland, you can get a slice of Haggis, battered and deep-fried.
When cold, Haggis gets very solid.
The import of Scottish-made Haggis into America has been banned since 1989, owing to the BSE (Bovine Spongiform Encephalopathy) problem in Britain. The USDA (United States Department of Agriculture) had felt that the sheep lung in Haggis, even though sheep aren’t cattle, might be an issue as well. In January 2010, the USDA said it would conduct a review of that 1989 decision, based on a ruling from the OIE (World Organization for Animal Health) that sheep lung is safe to consume.
Most food researchers have concluded that Haggis is actually not particularly Scottish in origin.
It is like one big sausage, using animal intestines (in this case the largest, the stomach) as a food wrapper and the cheapest cuts of meat to stuff it with, has been a common practice throughout history in order not to waste any food.
“The Scottish haggis may be an entirely indigenous invention, but in the absence of written records there is no way of knowing; it could be an adaptation of a Roman recipe to the local mutton and oats.” 
“Taken to be a distinctly Scottish dish from the 1750s onwards, the haggis can be shown to have an interesting derivation and the ‘evidence’ is cast across several different areas. It was extolled as an English dish by Gervaise Marcham in 1651.” 
“Whatever its origin and distressing though it may be to patriotic Scots, the word, like the dish, is definitely English.” 
In the West Country of England, in Wales and in Ireland, similar dishes are made — a filling of heart, liver, lungs and a grain. They use sausage casings or cauls instead though as the wrapper.
In Russia, there is a dish called “nyanya” or “niania” (meaning “nursemaid.”) It is described in Dead Souls by Gogol as a “well known delicacy that consists of sheep’s stomach stuffed with buckwheat, brains and sheep’s trotters.” It is served with cabbage soup.
Literature & Lore
“They served haggis at the last dinner I attended. I didn’t know whether to kick it or eat it. Having eaten it, I wished I’d have kicked it.” Stuart Turner
“The fact that I am not a haggis addict is probably due to my having read Shakespeare. It is the same with many Englishmen. There is no doubt that Shakespeare has rather put us off the stuff…. You remember the passage to which I refer? Macbeth happens upon the three witches while they are preparing the evening meal. They are dropping things into the cauldron and chanting ‘Eye of newt and toe of frog, wool of bat and tongue of dog,’ and so on, and he immediately recognises the recipe. ‘How now, you secret, black and midnight haggis,’ he cries shuddering.” — P.G. Wodehouse (1881-1975)
“My dear,” said Sobakevitch, “the cabbage soup is excellent.” With that he finished his portion, and helped himself to a generous measure of niania — the dish which follows shtchi and consists of a sheep’s stomach stuffed with black porridge, brains, and other things. “What niania this is!” he added to Chichikov. “Never would you get such stuff in a town, where one is given the devil knows what.”
“Nevertheless the Governor keeps a fair table,” said Chichikov.
“Yes, but do you know what all the stuff is MADE OF?” retorted Sobakevitch. “If you DID know you would never touch it.”
— from Dead Souls by Nikolai Gogol
 Davidson, Alan. The Penguin Companion to Food. London: The Penguin Group, 2002.
 Harrison, Alan. The Gastronomy of Haggis and Kail. Nutrition and Food Science Journal. No. 89, July/August 1984.
 Edwards, Gillian Mary. Hogmanay and Tiffany: the Names of Feasts and Fasts, London: Geoffrey Bles. 1970. Page 53.
Carrell, Severin. US haggis ban: ‘American Scots are the centre of the Scottish diaspora’: Severin Carrell on the US lifting Scottish haggis ban, imposed during BSE scare. Manchester: The Guardian. 25 January 2010. Web audio. Retrieved January 2010 from http://www.guardian.co.uk/uk/audio/2010/jan/25/haggis-usa-ban-lifted-burns-night
Dutta, Kunal. America’s long wait for haggis may be over. London: The Independent. 25 January 2010.
Scots attempt to overturn US ban on haggis. London: Daily Telegraph. 22 January 2011.