This phrase is typically used to express shock or surprise. While originally, it was generally used by people from the Greater
Boston area, it has since spread across the Eastern United States - making it as far south as Central Florida.
This phrase is not to be confused with "I cannot AHAHA oh my
god." While similar in use cases, "I am lowkey literally deadass dead" is used in instances where
humor may or may not be present, while "I cannot AHAHA oh my
god" is typically used in a comedy setting.