How are made up words for anime/manga formed (reading and kanji)?


EDIT: Eiríkr Útlendi has answered my question mostly in regards to readings.
What remains is how kanji are chosen for their meanings and why certain words
are selected over others e.g. 実 versus 果実. Also there are exceptions to the
rule e.g. 天竜人{てんりゅうびと}. How do those factor in?

I've been watching the Anime "One Piece". There are quite a few compound words
that obviously seem made up for the universe, and when I look them up the
constituent kanji either don't follow the readings they would as original
words [example 1] or they use a listed reading that doesn't occur in any words
I can find in my dictionaries [again example 1] (I use
[ejje](http://ejje.weblio.jp/) and [Jisho](http://jisho.org)). Otherwise,
there are words that use readings that appear more archaic or irregular to me
[example 2 and 4]. Then there are words that use the kanji and readings of the
original word (in example 3, "emperor"), but only part of it (in this example,
only one of the kanji of 皇帝 and thus only the reading from one kanji). Maybe
my vocabulary is too poor and I'm making mistakes because of that, but here
are the examples:

Example 1: 三{さん}刀{とう}流{りゅう} Translated as "three sword style" - This makes
sense, except why is とう and not かたな used?  
Example 2: 海{かい}楼{ろう}石{せき} Translated as "Sea Watchtower Stone" - 2nd makes
sense, but I don't understand why かい is used for the 1st and not うみ and せき (I
can't find any words with this reading) instead of いし for the 3rd.  
Example 3: 四{よん}皇{こう} Translated as "4 emperors" - Why just こう and not よんこうてい  
Example 4: 七{しち}武{ぶ}海{かい} Translated as "7 Warlords of the Sea" - Here かい
again. Also why use the irregular reading しち?  

Example 5: ゴムゴムの実 Translated as "Rubber Rubber Fruit" - Why not 果物 or 果実?

Is there a logic to these? Are they just made up to sound cool? If so, how
will I know when reading a new manga what these made up words mean? Someone
can definitely tell me that I'm overthinking this, haha.

Thanks for your input.




Made-up words are generally based on the existing Japanese naming convention.
I generally recommend that you familiarize yourself with a lot of Japanese
existing compounds before wondering about this problem at this level.
Especially distinguishing on- and kun- readings is critical. Please read [this
section](https://en.wikipedia.org/wiki/Kanji#Readings) carefully.

> 三刀流

If you already know a word
[二刀流](https://ja.wikipedia.org/wiki/%E4%BA%8C%E5%88%80%E6%B5%81), this should
look natural. さんかたなりゅう makes almost no sense to me. And See Eiríkr Útlendi's
answer. It's a simple on-on-on compound.

> 海楼石

The author just followed the standard naming convention of rocks. See:
[岩石の一覧](https://ja.wikipedia.org/wiki/%E5%B2%A9%E7%9F%B3%E3%81%AE%E4%B8%80%E8%A6%A7).
Basically all rocks are technically named as ～石【せき】 or ～岩【がん】. かいろう is a
simple on-on compound, which looks far more natural than うみろう.

> 四皇

四皇帝 makes sense, but both 皇 and 帝 already mean "emperor", so you don't have to
keep both of them. Similar things happen all the time. For example, 法律 ("law")
is a compound made of two kanji with the same meaning (法 = "law", 律 = "law").
Only 法 is used when it forms other compounds: 民法, 六法, 諸法. Another example is
道路 ("road"), which is made of 道 ("road") and 路 ("road"). There are many
compounds which only use 道 (国道, 車道, 歩道, ...). 四皇帝 is fine but seems less
interesting as a made-up word. And there are many two-kanji compounds where
only 皇 is used to mean "emperor/imperial": 上皇, 皇位, 皇籍

> 七武海

It's just a on-on-on compound. ななぶかい is not impossible but would sound a bit
strange.

> ゴムゴムの実

Read [Sino-Japanese vocabulary](https://en.wikipedia.org/wiki/Sino-
Japanese_vocabulary) first. There are many related questions on this site (for
example [this](https://japanese.stackexchange.com/q/17935/5010),
[this](https://japanese.stackexchange.com/q/28365/5010)). Basically 果実 sounds
more formal/technical than 実. In this case I feel 果実 wouldn't go very well
with ゴムゴム(の), which is another made-up word that sounds nothing
technical/formal.

> 天竜人

てんりゅうじん (yes, simple on-on-on compound) would perfectly make sense in modern
Japanese. As you probably know, じん the default suffix when we plainly refer to
イギリス人, 日本人, 宇宙人, etc. But ～びと was used a lot in the old days to refer to
"people of/in ～". So using びと adds the old/historical/traditional nuance to
it. For example, 外つ国人 （とつくにびと） is a word preferred in samurai dramas to refer
to foreigners.




Made-up words are generally based on the existing Japanese naming convention.
I generally recommend that you familiarize yourself with a lot of Japanese
existing compounds before wondering about this problem at this level.
Especially distinguishing on- and kun- readings is critical. Please read [this
section](https://en.wikipedia.org/wiki/Kanji#Readings) carefully.

> 三刀流

If you already know a word
[二刀流](https://ja.wikipedia.org/wiki/%E4%BA%8C%E5%88%80%E6%B5%81), this should
look natural. さんかたなりゅう makes almost no sense to me. And See Eiríkr Útlendi's
answer. It's a simple on-on-on compound.

> 海楼石

The author just followed the standard naming convention of rocks. See:
[岩石の一覧](https://ja.wikipedia.org/wiki/%E5%B2%A9%E7%9F%B3%E3%81%AE%E4%B8%80%E8%A6%A7).
Basically all rocks are technically named as ～石【せき】 or ～岩【がん】. かいろう is a
simple on-on compound, which looks far more natural than うみろう.

> 四皇

四皇帝 makes sense, but both 皇 and 帝 already mean "emperor", so you don't have to
keep both of them. Similar things happen all the time. For example, 法律 ("law")
is a compound made of two kanji with the same meaning (法 = "law", 律 = "law").
Only 法 is used when it forms other compounds: 民法, 六法, 諸法. Another example is
道路 ("road"), which is made of 道 ("road") and 路 ("road"). There are many
compounds which only use 道 (国道, 車道, 歩道, ...). 四皇帝 is fine but seems less
interesting as a made-up word. And there are many two-kanji compounds where
only 皇 is used to mean "emperor/imperial": 上皇, 皇位, 皇籍

> 七武海

It's just a on-on-on compound. ななぶかい is not impossible but would sound a bit
strange.

> ゴムゴムの実

Read [Sino-Japanese vocabulary](https://en.wikipedia.org/wiki/Sino-
Japanese_vocabulary) first. There are many related questions on this site (for
example [this](https://japanese.stackexchange.com/q/17935/5010),
[this](https://japanese.stackexchange.com/q/28365/5010)). Basically 果実 sounds
more formal/technical than 実. In this case I feel 果実 wouldn't go very well
with ゴムゴム(の), which is another made-up word that sounds nothing
technical/formal.

> 天竜人

てんりゅうじん (yes, simple on-on-on compound) would perfectly make sense in modern
Japanese. As you probably know, じん the default suffix when we plainly refer to
イギリス人, 日本人, 宇宙人, etc. But ～びと was used a lot in the old days to refer to
"people of/in ～". So using びと adds the old/historical/traditional nuance to
it. For example, 外つ国人 （とつくにびと） is a word preferred in samurai dramas to refer
to foreigners.

