A thincel is someone who is so thin that is has resulted in them being on the inceldom spectrum. Cybercels in the incelosphere will usually call them a volcel unless he/she has made active attempts to bulk up. As such, among incelospherians, the only legitimate thincel is one who has an abnormally high metabolism.
Having an abnormally high metabolism would result in a constant high fever from excess heat production, making it potentially life threatening. People who claim to have high metabolisms actually just have poor diets, a high carb low fat diet lets you "eat as much as you want without getting fat" because carbs are used by the body right away, thincels also tend to eat less in general, which means they hit their maintenance, often almost entirely with carbs, and then stop eating so no fat can be stored. They also tend to eat food with low overall nutritional value such as candy and cookies, which are saturating and lead to a feeling of fullness despite energy deficiency.
The solution here is simple, take in more energy than you expend, eat more nourishing foods such as eggs, rice, meat and beans and keep eating even if you're not hungry anymore, it is physically impossible for your metabolism to delete energy from existence so no matter how high you think your metabolism is, doing this will allow you to gain weight.