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.