Something one might say while making/retrieving toast, biscuits, soda bread, or another flour-based food item, with the intention of finding a suitable garnish. One might ordinarily use margarine or other butter substitute in such a situation, and, in fact, the margarine is right there next to the butter, but in this case one does not pick it up. No; one instead picks up the butter, saying, "fuck this, I'm having butter".
The use of butter here is pretty interesting. If one normally uses margarine, as practically everyone seems to these days, butter, as a seldom-used product, takes on the same luxurious quality it has in fairy tales, and actually did have up to the very recent past. You don't use it every day; it is uncommon although not exorbitantly expensive. You use it for making things like pastry, things that actually need butter in order to come out correctly. So when you use butter simply as a meltedy bit of flavor and texture on your bread, it is a small luxury, but still a luxury.
Historically, butter was a much more obvious luxury. Think fairy tales in which the poor family is overjoyed to discover they have the money to keep chickens. They can have EGGS! That story. The story in which on your birthday you are the only one to get a thin scraping on your breakfast bread. If you are ill they save it for you and everyone else goes without. The story in which you can't afford to keep a cow, so you don't get butter; you get your fat from the meat you hunt. You have dripping. Think the Little House on the Prairie series, think The Long Winter. When you are living day to day to day and you render your own fats so you can cook at all.
Think the Great Depression, during which no one could afford anything. You were lucky to get bread, let alone anything on it. Think World War II and rationing, especially in England. Think sending all your farm milk off to help the effort. Think of owning a dairy in that time, of sending cans on cans off in trucks to the government every morning. Drinking skim milk is not as a healthy alternative to whole milk, but an economic necessity. How can we get the most from our limited supply? What can we stretch, what can we give up? You end up saving the milk for the baby. You save the butter for the old and sick. Everyone else gets oleomargarine. People really don't seem to like margarine much, but it is the substitute available. Even after the war, they keep using it; no one has any money yet, and it is cheaper than butter. It fulfills the same function. People may miss the taste of butter, but they can accept going without.
This mindset has changed since WWII, yes. No one in the USA is worrying too much about finding butter in the stores. It's not prohibitively expensive; four sticks of butter run you something like $2.50. This is more than margarine, at about $1, but the extra $1.50 is something most people can handle. It's not just a money issue anymore. Instead it has become a health issue. Butter has lots of saturated fat; margarine has less. So health-conscious people use margarine on an everyday basis. Butter then becomes a treat you give yourself. You do not rely solely on margarine: you use butter for complicated recipes, foods which really require butter to come out properly. But you also know how much fat there is in the finished product. You know that butter, while making a given cake taste so good, also makes it bad for you.
So the taste issue comes back as well. In short, many people prefer the taste of butter to that of margarine. So, if you combine this with health consciousness, you get a huge wave of butter substitute products on the market, brilliant with advertising dollars. Margarine just does not taste like butter; I Can't Believe It's Not Butter, in contrast, apparently tastes an awful lot like butter. And it's low in fat! Kids love it! Chicks dig it! Even Fabio agrees.
The really interesting thing here is that people want the taste of butter -- a STICK of FAT, you realize -- with the least possible fat content. So companies pour research into chemical substitutes, creating tubs of special whipped butter product with god knows what in them. You are substituting chemical additives for fat; how much healthier is this anyway? It seems to be a psychological issue. Butter is fat. Butter products are low-fat or non-fat. But they taste like butter, right? They are seen as healthy, while butter is unhealthy. Yet they have the same qualities which we currently value in the fat for which they substitute. They are created to be as much like actual butter as possible, without the negative aspect that butter itself holds.
Thus we come back around to the opening sentence: "fuck this, I'm having butter". When you no longer care about the health issues involved in eating fat, when in fact you are a very healthy person, possibly even an underweight person who really needs their daily fat, when you set aside all the societal preconceptions and health awareness and just concentrate on taste, this is when you can use this sentence. You don't care about the plethora of products surrounding butter; you care about the butter itself, how you rub it into the perfect pie crust, how it provides the perfect layer of contrast in a jam-laden biscuit. You care about your mouth, your tongue, your taste buds. The next time you have a piece of toast, you will probably go back to your margarine; no one ever said the financial issue had completely disappeared. But for now, you can afford it. You have your butter, and no one can touch you.