Anne-Marie B. answered 07/02/19
College Counselor, English Teacher, Violin Teacher
In the case of the modifiers (so, very, etc) before the gradable or ungradable adjectives, the modifiers are needed for hot because you wanted to highlight the extremeness of the heat. Boiling by definition already implies a very high level of heat. No need for modifiers, and actually a quicker, more efficient way of expressing the idea.
Terms like SO dead don't need explanation. It is not a matter of grammar or correct or not correct. It is simply "slang" and just an expression we use. Also, in this "expression", the word dead is not being used literally so there is no need to treat it as such (with proper modifiers).
Same thing with very pregnant. Of course it is not correct or an accurate modifier for pregnant, but just an expression that we use, and every one knows we probably mean 7 to 9 months pregnant or just implying that she is very big (as opposed to newly pregnant).