Using "so" and "very" for ungradable adjectives
We generally use modifiers such as "so" and "very" for gradable/normal adjectives (water can be quite/so/very HOT, but not quite/so/very BOILING (an ungradable/extreme adjective). Yet would you say the following sentences--which, I'd say, are quite commonplace in colloquial speech--are grammatically incorrect?
You told the teacher I'd been cheating? You're SO dead! ("Dead", obviously, is ungradable.)
Simon Cowell was seen with his VERY pregnant girlfriend. (A woman is either pregnant or not--it can't be graded.)
Solution 1:
Sometimes words you may consider not gradable are used as gradable.
"Very pregnant" means last months of pregnancy, belly extremely bulged, movement impaired - a girl within first trimester can work at most jobs just fine. One who is very pregnant needs a lot of help.
When you simmer or boil your pasta, it's just boiling. If the water splashes all over the stove, or the cover is jumping on the pot, it's very boiling and you should reduce heat to let it simmer.
When you broke your mother's vase, she isn't going to commit a murder. Still, you're "so dead!" - meaning you're in a lot of trouble. In this case "dead" is used as hyperbole, synonymous to "in trouble" and that, in order is perfectly gradable.
Solution 2:
It depends on if you define grammatical as conforming to the rules of grammar or regarded as correct and acceptable by native speakers of the language.
I believe everybody has heard those phrases, and probably they have raised the hackles of some. However, I think they are acceptable on the basis of the second definition of grammatical.
We understand that in most instances, so dead is a figure of speech. As such, as an idiom, it is grammatical, meaning in deep trouble, or worse, a threat:
Arrested: Hope Williams, 14, left, allegedly wrote 'he is so dead' about a classmate she believed was a snitch.
The same applies to very pregnant as an idiom.
Thirty-five-and-a-half months pregnant is very pregnant indeed. (correct)
The irony as regards the world's demise is very pregnant indeed. (correct)
So at 4 weeks one is very pregnant indeed. (I would say this was not idiomatic and therefore a rather silly use of very pregnant.)
Solution 3:
Both can be defended.
In the first, so is used for emphasis, and, in any case, dead isn’t being used literally.
In the second, very is also used for emphasis. The OED’s third definition of very is:
In emphatic use, denoting that the person or thing may be so named in the fullest sense of the term, or possesses all the essential qualities of the thing specified.
It means that her pregnancy was apparent, as opposed to that of a woman who had only recently become pregnant.