Why is the honorific o used for the bathroom?

Question

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?

Answer 1

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:

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.

Answer 2

Do you mean o-cha instead of o-chai?

Also, 'o' can definitely be used with "o-tearai". For the longest time, I had only heard it said that way and didn't realize you could drop the 'o'. In the case of o-tearai, as in other cases, it is being used to make the utterance more polite. The talk about holiness in the first link is kind of distracting from the fact that 'o' just makes speech sound more polite or refined. It certainly wouldn't mean "holy-vagina", and o-manko is still not considered polite. Honestly, I'm not sure if people would also just say "manko" or if differentiating between the two would make a difference.

I think you should just rephrase "Is it sometimes used to soften things that are somehow 'dirty'" to "It is used to soften things". Dirty things are no exception.

There's some history behind it that I don't really know enough abuot to get into, so I'll leave that for someone else.