noun
love-sickness
A set phrase literally meaning 'outside the four hundred and four diseases', referring to love-sickness as the only ailment not covered by traditional medicine. Used in classical or literary contexts.
昔の人は恋煩いを「四百四病の外」と呼んだ。
People long ago called love-sickness 'shihyakushibyō no hoka'.
Modern, common word for love-sickness. 四百四病の外 is a classical set phrase with the same meaning.
From the Buddhist concept of 四百四病 (four hundred and four diseases) representing all human ailments, plus の外 (outside), implying love-sickness is beyond medical treatment.