jukujikun 熟字訓

In Japanese, a jukujikun 熟字訓 is a special type of kun'yomi kanji reading, literally a "kanji compound kun [reading]."

It mostly happens when a single morpheme is written with multiple kanji instead of one. For example:

hito 人
Person
Kun'yomi, one morpheme, one kanji.
jinsei 人生
Life [of a person].
On'yomi, two morphemes, two kanji.
otona 大人
Adult.
Jukujikun, one morpheme, two kanji.

The consequence of a jukujikun reading is that it's impossible divide the reading of the word in two. For example, you can divide jinsei 人生, which has two morphemes, into jin 人 sei 生. But you can't divide otona into oto 大 na 人 or o 大 tona 人.

Ye Olde Nihon

For the most part, what are called jukujikun are old Japanese words which were going to be written with kanji, but couldn't match the meaning of a single kanji, so they had to be written with multiple kanji instead.

For example, there were two Japanese words, ima, "now," and "day," hi. Since there was a kanji that meant "now," the word for "now" in Japanese got associated with it: ima 今. And there was a kanji for "day," too, so the same happened with it: hi 日. This way, ima and hi became kun'yomi readings of those two kanji.

Then, there was a word for "today," kyou. But... wait?! Where's the kanji for "today?" There's no kanji for today! Oh no! What do we do! What do we do?! We imported all these bucket-loads of Chinese characters and just like with legos you can't find the one you want when you need no matter how many you got!

Oh well, I got the kanji for "now" and "day," and "today" is kind of "now-day" anyway, so... kyou 今日. That's it. One old Japanese word associated with two kanji instead of one. A jukujikun. Problem solved, folks, let's go home!

And that's how jukujikun were born.

Multi-Morpheme

Like I've said, most jukujikun are single-morpheme words, however there are some exceptions.

For example, kesa 今朝, "today's morning," is a jukujikun. However, it's, very obviously, the fusion of kyou 今日, "today," with asa 朝, "morning," in Japanese. I wouldn't know for sure, but I guess you could say they are two morphemes? Or were? They just... kind of gotta blended together, their pronunciation.

Anyway, this shows that sometimes two old Japanese words become one old Japanese word that's written with two or more kanji creating a jukujikun reading that can't be divided in half.

On'yomi

I think it's important that sometimes words read by the kanji's on'yomi have similar meanings to the jukujikun of the words.

For example, konnichi 今日 is the on'yomi version of the jukujikun kyou 今日. Unlike kyou, konnichi can be divided into two: kon 今 nichi 日.

Le Neo Japan

Besides old words, jukujikun may also refer to modern words associated with multiple kanji.

This happens specially with words loaned from the west, the gairaigo 外来語, which are written with katakana and do not have kanji.

When a katakana word like that is forcibly written with kanji, there are two options: the kanji are chosen phonetically, this is called an ateji, or by meaning, this is called a gikun.

For example, tabako タバコ is a foreign word which may be written with kanji, like this: tabako 煙草. Since it's a single word written with two kanji, it's a jukujikun, and because the reading is "artificial," it's a gikun.

Examples

For reference, some examples of jukujikun.

tsuitachi 一日
First day of the month.
futsuka 二日
Second day. Two days.
ashita 明日 (or asu 明日)
Tomorrow.
kinou 昨日
Yesterday.
kotoshi 今年
This year.
chotto 一寸 (rarely written with kanji, though)
One bit. One little. One moment.
(often means, "wait!" or "stop!")