The phrase is from Suwen-Zhizhenyao Da Lun. "Severe" refers to a complex and difficult-to-recognize condition that is serious. For example, extreme heat resembling cold (true internal heat with false external cold) or extreme cold resembling heat (true internal cold with false external heat). One should follow the false appearance of cold or heat and adopt a method generally referred to as "paradoxical treatment," known as "congruent treatment."