Why does 留守{るす} have two almost opposite meanings?

Question

Why does 留守{るす} have two almost opposite meanings? One is "to keep the door" or "to look after home" and the other almost opposite meaning is "not at home".

Answer 1

留 means stay, and 守 means guard, so 留守 literary means "stay and guard (home)". It actually used to mean just like this in the old Japanese. 留守 referred to keeping the house while the master is away from home.

However, such usage is dying or at least very literary now. The main meaning of 留守 in modern Japanese is "not at home". "留守にする" always means "be away (from home, office, etc.)".

• 留守の語源

  本来の意味は、主（あるじ）不在の家を守ることであったが、不在の主の方に重点が移り、 単に「不在」の意味に使われるようになった。また、もとの意味の「守り」は、「留守番」「留守居」と呼ぶようになった。

• 留守（るす）・・・・もともとは「不在」という意味ではなかった

  留守という言葉は、中国語に由来していて、「皇帝が都を不在にしている間、都に留まって代わりに政治を行う」という意味があり、そこから「主（あるじ）が不在の家を守る」という意味になった。
  やがて、主がいない間を守るという意味よりも「不在」ということに意識されるようになり、鎌倉時代には、現在と同じく単に「不在」の意味で「留守」という言葉が使われるようになった。

If we want to say "to look after the house", we usually use "留守番(をする)". Here, 番 roughly means "keeper" or "caretaker".

Very rarely, expressions like "Aが城の留守をしている" ("A is looking after the castle") are seen in recent novels. There is also a word 留守部隊, which means the party who don't go to the front line. But I think those expressions can be understood only with the help of the context. Using 留守番 is almost always the safer choice.

In some certain idioms, 留守 and 留守番 are still interchangeable and you can safely use 留守:

• 留守を頼む ≒ 留守番を頼む ask someone to look after the house
• 留守を任せる ≒ 留守番を任せる trust someone to look after the house

Answer 2

I think the main point to look at are the usages of "を" and ”に"

"を" is suggesting that the action is done so "留守をする" is the original meaning of looking after. "に" is suggesting direction, in other words to put in the state of the action. So in this way "留守にする" can be considered to mean "to put in a state where somebody else is looking after it" and hence could easily evolve towards just meaning "put in a state where somebody(you, etc.) is not there".

I also have to note that from experience Japanese doesn't seem to be grammatically strict and that abbreviations are common place which would probably contribute.