Why is the honorific o used for the bathroom?


I've seen the honorific "o"/"go" (is it called bikago?) being used as
politness or reverence: o-cha for non-western tea, o-namae when talking about
someone else's name, o-genki instead of just genki, and o-tera for Buddhist
temple.

However, I've recently come across (in "Welcome to Japanese") o-tearai for
bathroom, which is presumably a modification of tearai.

Google-sensei directed me to <http://www.learnjapanese.com/japanese-o-factor/>
and <http://www.peterpayne.net/2008/05/useful-japanese-honorific-o.html> ,
which talk about it being used for dirty things, such as a potty, diapers and
female anatomy.

Is it sometimes used to soften things that are somehow "dirty", or is it being
used ironically?




As you suspect and Nathan writes, softening the nuance may be one factor, but
there is another factor. Without `o-`, the underlying form is `te-araw-`,
which ends with a verb stem `araw` (later, the epenthetic vowel `i` is
inserted, and `wi` changes to `i` , which is not crucial). Even though a verb
stem can be used as a noun, it is often not stable as a noun. Addition of `o-`
to an expression ending with a verb stem makes the expression unambiguously a
noun, and stabilizes that expression as a noun. Sometimes, there is no version
of the word without `o-`. This case is one example, and although you can say
`手を洗う` as an ordinary verb phrase, you cannot say `手洗い` as a noun. Other
examples can be found in my answer to [this
question](https://japanese.stackexchange.com/questions/2292/does-anyone-know-
of-any-o-words-or-go-words-which-are-absolutely-neutral):

> おむすび  
>  おこぼれ 'something positive gained (unexpectedly) from someone else' [Not
> falling off]  
>  お裾分け 'a portion given away'  
>  お下がり 'used thing (clothes, etc.) often given from a senior to a junior
> sibling' [Not going down]  
>  おあずけ  
>  お手上げ  
>  お手付き

All these expressions require `o-`. The version without it does not exist.




As you suspect and Nathan writes, softening the nuance may be one factor, but
there is another factor. Without `o-`, the underlying form is `te-araw-`,
which ends with a verb stem `araw` (later, the epenthetic vowel `i` is
inserted, and `wi` changes to `i` , which is not crucial). Even though a verb
stem can be used as a noun, it is often not stable as a noun. Addition of `o-`
to an expression ending with a verb stem makes the expression unambiguously a
noun, and stabilizes that expression as a noun. Sometimes, there is no version
of the word without `o-`. This case is one example, and although you can say
`手を洗う` as an ordinary verb phrase, you cannot say `手洗い` as a noun. Other
examples can be found in my answer to [this
question](https://japanese.stackexchange.com/questions/2292/does-anyone-know-
of-any-o-words-or-go-words-which-are-absolutely-neutral):

> おむすび  
>  おこぼれ 'something positive gained (unexpectedly) from someone else' [Not
> falling off]  
>  お裾分け 'a portion given away'  
>  お下がり 'used thing (clothes, etc.) often given from a senior to a junior
> sibling' [Not going down]  
>  おあずけ  
>  お手上げ  
>  お手付き

All these expressions require `o-`. The version without it does not exist.

