If only the topic of kanji readings could stop at, kanji would be much simpler to learn. In reality, however, Japanese words have a lot of exception classes and phonetic situations that don’t conform to a one-size-fits-all rule.
One of these situations is ateji (当て字) readings.
当て字 are arbitrarily assigned special readings of kanji compounds that follow either one of these two distinct rules:
- Use kanji phonetically in foreign borrowed words because of their sound/reading, irrespective of their actual meanings;
- Use kanji semantically due to their meaning, irrespective of their actual pronunciation/readings.
It’s good to point out that these two rules are basically the opposite of each other. And yet, both of these categories get commonly referred to as 当て字.
For the first class of 当て字, kanji used for their sounds, we have some interesting words like 亜細亜 (あ + じ + あ = アジア = Asia) or 亜米利加 (あ + め + り + か = アメリカ = America).
As you can see, in this usage, the phonetic role of the kanji is important, and for this reason it’s common to write the furigana on top of these words as katakana.
The practice of using kanji like this has been mostly superseded in modern Japanese thanks to the usage of katakana for loan words, but some of these still remain floating around albeit not as commonly anymore.
On the other hand the second class of 当て字, called jukujikun (熟字訓), is still very much in use today.
One example is 今朝 which is written with the kanji 今 (now) and 朝 (morning) to mean “this morning”. However, neither of these kanji alone have any readings that would map to けさ: the word itself “けさ” as “this morning” only exists as its own standalone unit formed by those two kanji in an inseparable manner.
Other similar words are 大和 (Japanese ethnicity) with the kanji big (大) and japan (和), and 煙草 (tobacco) with smoke (煙) + grass (草) (although you’ll often see it written in katakana alone).
When it comes to these 熟字訓 the only way to know how they are pronounced is to look them up in a dictionary, because the kanji themselves will not give you any certainty on their reading. (See also:)