What does 乃 mean here? On a particular forum is a banner reading:


  此乃舊貼唯讀區


which I imagine is a highfalutin way of saying that this thread is read-only.

Now, 乃 has many senses in my dictionary. What is the sense of 乃 being used here? My best guess would be "only", in the sense that this page is "nothing more than" a read-only area.
Here "乃" means "be". This sentence means "This area is for old posts, and these posts are read-only."

Next time, if you find "此乃" these two characters together or a noun+乃 together, "乃" properly means "be". For example:


  此乃我的书.
   This is my book.
  
  意识乃是大脑的一种自然属性。
   Consciousness is an emergent property of the brain.