Etymological origin of -n in 洗 (セン)


I've always wondered why the 音読み of the kanji 洗 is セン when, in all Chinese
topolects I'm aware of, the reading ends in a vowel.

I always chalked this up to Japanese having borrowed a Middle Chinese reading
at a time when 洗 still ended in a nasal (cf. the Japanese vs Chinese reading
of 鳥), but I was surprised to find that this isn't the case, and that it's
reconstructed as *sejX in Middle Chinese.

Why, then, is there a nasal in the Japanese reading (and according to
Wiktionary, the Korean reading too)?




I assume you're referring to [the Baxter-Sagart Middle Chinese
transcriptions](http://en.wiktionary.org/wiki/Appendix:Baxter-
Sagart_Old_Chinese_reconstruction) of the [Qieyun rime
dictionary](http://en.wikipedia.org/wiki/Qieyun). When I search these
transcriptions for 洗, I find _two_ readings listed:

  1. *sejX, corresponding to modern Mandarin xǐ, Japanese セイ・サイ, and Korean 세 se 
  2. *senX, corresponding to modern Mandarin xiǎn, Japanese セン, and Korean 선 seon

So I think the final nasal already existed in Middle Chinese and was borrowed
into Japanese. This may not have been obvious because xǐ is the usual reading
in modern Mandarin while セン is more common today in Japanese.

As for xiǎn, I don't know when this reading is used, but it _is_ at least
listed in my dictionary. My Chinese is too rudimentary for me to say any more
on the subject. The Japanese and Mandarin readings listed above are from 新漢語林,
and the Korean readings are taken from Wiktionary.

(For those who are curious, the final X in Baxter's notation indicates [a
rising
tone](http://en.wikipedia.org/wiki/Baxter%27s_transcription_for_Middle_Chinese#Tones).)

