What does "I so am." mean? Is it a common expression?

Native speaker’s answer
Rebecca
"So" in this context is used to mean "really". So "I so am" means "I really am". "I so am" is not a general expression, but the use of "so" in this way is common. However "so" is a lot more informal than saying "really", and is not as common to use in this way. Ex: A: He is handsome. B: He so is.