I heard this on TV, a woman got very angry with a girl after the girl said something about her. The woman was shocked to hear that, wide opened her eyes and said to the girl:
"You are so lying."
I didn't know that "so" could be used with verbs to mean "shockingly too much". So, I wondered maybe I could incorporate it in my speaking skills. So I wondered if I can use "so" with any verb. For instance, can we say
You are so drinking.
You are so shouting.
You are so sleeping.
Nobody should have an issue when you use 'so' with gradable adjectives, such as "you are so dirty!" because one can be a little dirty, or very dirty. It stands out as unusual when the word it modifies is not gradable. If you view the word 'so' in your example as an adverb of degree operating on the verb "lying" then it sounds technically wrong because you cannot have gradations of "lying" - one either is or is not lying. But this has become a common use, accepted by (most) English speakers as a colloquialism, especially younger people. It was something of an Americanism, but over time British English speakers have adopted it as well. It is used, not to grade the word, but to emphasise it.
Having said that, 'so' has a secondary usage to mean that something previously mentioned is true, as seen in phrases like "so it was that". Children locked in a battle of contradiction over something one has said will often repeat "is not!" which is countered by "is so!". Arguably, saying someone is "so lying" is a reasonable comeback to the denial that they "are not lying".