What is the meaning of しも?

Question

In a recent class, my Japanese teacher taught me the use of しも as something akin to a formal version of でも as in the following sentences:

• いつでも暇{ひま}じゃないんですよ。
• いつしも暇{ひま}じゃないんですよ。

I was told the above sentences have the same meaning, varying only with formality.

Now, when she wanted me to practice using しも, she gave an example that uses it in a completely different way from what she explained. She gave the following sentence:

母{はは}の考{かんが}えが必{かなら}ずしも正{ただ}しいとはかぎらない。

I understand the meaning. But I don't particularly understand what the use of しも is in this sentence. For me, the meaning stays the same whether or not しも is there (though this might only be because I don't really get it). My teacher explained it simply as an emphasis and told me I should try making sentences using しも like this on my own. But I don't see how I can try making up sentences when I don't really understand what it's for. No matter how much I search for しも, there is no explanation for it used as an "emphasis".

I tried searching about 必ずしも and I found results saying it means "not always (necessarily)" and that makes sense. Does this mean then that しも acts as some sort of negation? If so, is it just attached to adverbs? How is it formed/used? If not, what is it then?

Answer 1

I'm afraid to say that いつしも is very rare, and you can use it naturally only in literary sentences. ～しも was a grammatical element in archaic Japanese (it was a kind of intensifier similar to nothing but, even, necessarily, etc), but in modern Japanese it's not possible to combine an arbitrary word with it.

You will find しも only in the following fixed expressions.

• 必ずしも(～ない)
• まだしも
• 折しも
• 誰しも (emphatic version of 誰も)
• なきにしもあらず: see this question

These are relatively common, and basically it's better to memorize them individually. The usages are hard to generalize.

いつしも is a rare emphatic version of いつも. Actually, it's so uncommon that I had to google to check if it is really used in modern Japanese (FWIW there is zero result in BCCWJ). There are even rarer words like いましも, なおしも, and さしも, but these are just emphatic variations of いま, なお, さ(=そう), respectively.