TL;DR: swear words like “shit” usually emphasize the mood of a sentence rather than add new meaning, which is why “shit” seems to change connotation across your examples. Think like the word “very”.
Traditionally in most European languages, the cycle of what is considered most offensive shifts between bodily functions (shit, piss), genitalia and sexual acts (cunt, cock, fuck) and religious profanity (hell, bloody), particularly against the Christian God. Some scholars define us as moving into a new cycle, where the most offensive words are slurs based on race, sexuality, or gender. These scholars speculate that this results in more willingness to experiment with already existing swear words of the ‘traditional’ categories since they are considered less offensive in comparison.
Swear words are almost always used euphemistically and in set phrases. Some scholars go as far as to argue that swearing is only euphemistic and words used literally do not count as “swearing”. In fact, much of swear word usage can be classified as an intensifier, which is a word or short phrase that sort of heightens the already existing mood of the sentence but doesn’t explicitly change the meaning. Using shit as an example, “Shit, the bread’s gone stale again.” In this example, you can also see shit being used as a sort of mood marker, since it is reasonable to assume out of context that the bread going stale might be desirable to the speaker. In this case, the “shit” marks the stale bread is actually bad as well as intensifying the mood as compared to, say “oh darn, the bread’s gone stale again.”
This part is a bit of speculation on the origin of set phrases like “ain’t shit” or “the shit” and I haven’t actually read any scholarly literature on this topic specifically. You can see similarities between set phrases like “this is the shit” and “this is the stuff” and “that’s the spot”. It seems like this is a construction common in the English language to express that something is pleasurable. Whether this is what caused the “this is the shit” set phrase or whether the “this is the shit” set phrase caused the construction remains to be seen. “Ain’t shit” is definitely somewhat different because it probably comes from African American Vernacular English (AAVE). While I would argue that “the shit” could be AAVE in origin as well, I do think it has roots in non AAVE English, whereas “ain’t shit” is grammatically AAVE.
If you’re interested in reading more, I recommend The F Word by Jesse Sheidlower and Holy Sh*t by Melissa Mohr.
Hope my answer doesn’t get buried and I hope you don’t feel too overwhelmed by all the responses you’re getting. But something I found really useful is frozen veg. If you’re struggling to plan healthier meals that are higher calorie, frozen veg is a game changer. It doesn’t go bad, it’s cheaper than the fresh stuff, and the most important thing is you can add it to your existing diet. I have a soft spot for ramen and box mac and cheese for example, and it’s so easy just to throw handfuls of whatever I’ve got in the freezer into a pot of pasta or ramen to make it just a bit healthier. Hell, you can even forget the ramen altogether and just use the soup base (it’s just stock!) to make lazy soup. Add a chopped onion if you’re feeling fancy and that’s that.
You also list a lot of protein sources that you can’t afford to add to your diet. Protein is a necessary nutrient, but it’s not the end all to a healthy diet. I say that as a lifelong athlete. It’s very easy to get an appropriate amount of protein from plant based sources, and they tend to be a lot cheaper. Plus, they tend to be higher in other macros and nutrients. Soy milk, for example, has the same protein content as dairy milk (but might be more expensive depending on your area). Beans and other legumes are fantastic and tasty. Chickpeas are my favourite. If you have a blender or food processor, you can make hummus very easily. Lentils are also amazing if you are able to cook. Cheap as hell if you buy them in bulk and insanely filling. Indian dhaal is a lentil stew that’s fairly easy to make and very tasty. If you can afford it, snack on nuts and seeds. Add peanut butter (look at labels to find some that doesn’t have sugar in it) to your diet. Both those things are higher in calorie while also being high in nutrients. If you eat rice, try getting brown rice instead of white rice. It’s higher in protein and fiber and will likely keep you full for longer. Potatoes and other root vegetables are also awesome. Versatile, cheap, relatively high calorie, easy to cook, and keep for a long time if stored properly. I like to make a huge pot of potato stew with beans and frozen vegetables and keep it in the fridge for easy meals for like a week. If you’re looking for animal protein, check your local grocery for frozen fish. Its usually half the price of the fresh stuff.
I’m not your doctor, but personally, 800-1000 cal/day was terrible for my health. Yeah, it’ll make you lose weight, but for me it made me really lethargic and gave me brain fog. It just wasn’t enough to keep my body going. Maybe try slowly lowering your calorie intake and see how your body feels. I’ve also found that in the past, calorie counting was actually counterproductive to my health because what ended up happening was it became a “game” to eat fewer and fewer calories a day. Luckily I saw that and stopped counting calories before it turned into an eating disorder. My point here is just that it’ll take some work figuring out what works for you and don’t get discouraged if a method doesn’t fit your body or your lifestyle.
In terms of exercise, I know it’s not a satisfying answer, but it’s really going to depend on your body and what type of exercise you’re doing. If you’re exercising, you should definitely be eating more than 800-1000 cal/day unless you’re like, a toddler. It’s dangerous in my non-professional opinion to exercise when you’re under eating by that amount particularly if you’re lifting weights or doing high impact cardio.
I wish you luck on your journey and I hope it all works out for you :)