Etymology of 姪 and 甥


姪 "niece" is often pronounced [[me.i]] rather than [[meː]] (see e.g. [this
comment in
chat](http://chat.stackexchange.com/transcript/message/24426515#24426515)). I
think the reason for this is that there is a morpheme boundary in 姪,
presumably

> 姪 = 女【め】 + イ = female イ

Unfortunately, 姪 is not listed in any of the answers in the question "[Kanji
for native Japanese concepts: Kun'yomi spanning multiple
morphemes](https://japanese.stackexchange.com/q/6581/1628)" (or it might be
hidden in another word and I haven't noticed).

What is the meaning of イ and what is the etymology of 姪 and 甥?




Shogakukan's 大国語辞典 shows that 姪【めい】 has a historical hiragana spelling of めひ,
not めい, showing that the modern _mei_ reading is not _on'yomi_ but rather
_kun'yomi_. This different derivation is probably also why the pronunciation
is different: `[mei]` with a more distinct `[i]`, and not `[meː]`. The term
for "nephew", 甥【おい】, has a historical hiragana spelling of をひ. As you note, め
is an old morpheme indicating "female", and を is an old morpheme indicating
"male". We now have a clear follow-on morpheme ひ.

This ひ is of uncertain derivation. The closest vaguely relevant match that I
can currently find is prefix 曾【ひ】, often glossed as "great" in terms like
曾【ひ】お祖母【ばあ】さん "great-grandmother", 曾【ひ】孫【まご】 "great-grandchild", etc.
Shogakukan's given definition for 曾【ひ】 is:

> 血縁関係を表す語について、それよりさらに一代離れた関係にあることを示す。  
>  For terms expressing a blood relation, this indicates a relation separated
> by one more generation.

I wonder if there might have been an older meaning, still referring to
relative distance from oneself, but not necessarily of a different generation;
and an older usage, not as a prefix but as a noun. I'll keep looking.




Shogakukan's 大国語辞典 shows that 姪【めい】 has a historical hiragana spelling of めひ,
not めい, showing that the modern _mei_ reading is not _on'yomi_ but rather
_kun'yomi_. This different derivation is probably also why the pronunciation
is different: `[mei]` with a more distinct `[i]`, and not `[meː]`. The term
for "nephew", 甥【おい】, has a historical hiragana spelling of をひ. As you note, め
is an old morpheme indicating "female", and を is an old morpheme indicating
"male". We now have a clear follow-on morpheme ひ.

This ひ is of uncertain derivation. The closest vaguely relevant match that I
can currently find is prefix 曾【ひ】, often glossed as "great" in terms like
曾【ひ】お祖母【ばあ】さん "great-grandmother", 曾【ひ】孫【まご】 "great-grandchild", etc.
Shogakukan's given definition for 曾【ひ】 is:

> 血縁関係を表す語について、それよりさらに一代離れた関係にあることを示す。  
>  For terms expressing a blood relation, this indicates a relation separated
> by one more generation.

I wonder if there might have been an older meaning, still referring to
relative distance from oneself, but not necessarily of a different generation;
and an older usage, not as a prefix but as a noun. I'll keep looking.

