The etymology of 便当 in Chinese
I try to make a rough translation here:


  便当， originated from the Southern Song Dynasty, meaning "convenient
  things, convenient, smooth". 
  
  When introduced into Japan, it became “便道”、“辨道”、“辨当”. 
  
  Later, when the term “便当” was transplanted back to China from Japan,
  it's referenced to 弁当(bentou/bento). 
  
  The term 盒饭, (that is 盒装餐食), is used in mainland, while
  便当 is usually used in Taiwan. 
  
  However, there is a nuance between 盒饭 and 便当. 便当 is more delicate than
  盒饭.  盒饭 is usually just simple and normal food.