I eat all the time. I just have a high motabolism and I actually like being really skinny tbh lol.
I use to be really really skinny. I am still skinny (my collar bones are protruding, but they are naturally like that) people use this to say I am underweight
The BMI is kinda junk, though, cuz it doesn't differentiate between muscle weight and fat weight. There are Olympic champions who would be categorized as obese according to it.