Looking up some info on Godzilla, and it mentioned that it derives from the Japanese words for ‘gorilla’ and ‘whale’, (‘gorira’ and ‘kujira’, respectively [eh, more or less]).
‘Gorira’ is pretty obvious importation of the foreign word ‘gorilla,’ but what bout ‘kujira?’ Not knowing any Japanese I’d guess it means something like ‘really big fish.’ So what is it?
The word ‘whale’ in English doesn’t decompose to anything like ‘really big fish’. Why would you assume it does in Japanese? (It doesn’t, for the record).
Kujira means “whale”. What else did you think it might mean?
Or do you think the island nation of Japan was unfamiliar with whales before developing language? 
The etymology is unclear. Some speculate it came from “big mouth”, or “black and white”, but all we know for sure is that the word means “whale”.
Thank you. The Japanese etymology was what I was looking for, not the literal translation. My OP was vaguer in print than it was in my head.
There is no etymology. The entire Japanese language sprang fully-formed from the tongue of the first Sun-King a couple thousand of years ago, except for stuff like ‘tobacco’ and ‘ice cream’.
Regarding whale: ultimately from PIE *(s)kwal-o-, which is the source for Latin squalus: large sea fish. This also led to walrus.
Note that Greeks and Romans considered whales “fish”. So the Japanese may well have not had a distinction either.
Regarding gorilla, in case anyone is wondering:
Some crazy Carthaginian dude called Hanno went on a sailing expedition along the coast of Africa, and ran into some weird hairy ladies/apes/monkeys (it’s not entirely clear) that the locals called “gorillae”. Considering that some of them ended up killed and flayed, with their skins brought back to Carthage, after the (apparently desperate) Carthaginian sailors tried and failed to pull some moves on them (or something), I sure hope that they were apes. But there is a very slim chance that the word means “fugly local chick” in some ancient West African language.
Not a joke, BTW. Look it up.
The kanji (Chinese character imported into Japanese) for “whale” is composed of elements that by themselves mean “fish” and “capital”. That makes some sense, but doesn’t give any insight into why the Japanese pronounce it “kujira”. There is no Japanese word pronounced “jira”, and those pronounced “kuji” don’t have any meanings related in any way to whales. Thus there is no obvious etymology of the word. There’s just a word that means “whale”.
Adding on to what glowacks said, when the Japanese adopted the Chinese characters, they assigned their own words to the characters sometimes, and preserved the Chinese pronunciation sometimes. More often, they use the traditional Japanese word for single character words, and intermix between traditionally Japanese and traditionally Chinese words when they use multi-character words.
So the word “kujira” will have predated the Chinese character and won’t have influenced the design of the Chinese character. But, since the Japanese don’t have a writing system before adopting the Chinese one, there’s no way to track the history of the orally-transmitted word, kujira.
On the Chinese side, if “whale” was added to the lexicon sometime later, you could possibly find some etymological history to it more interesting than just “capital whale”, in how the pictographs morphed over time, but that won’t tell you anything about kujira.
According to the word in question on Wiktionary, the kanji (鯨) has an older realization/pronunciation. There’s also the word isa which just originally meant “powerful.” Sometimes they added a suffix and make it isana, which means “powerful fish.” There was also an old epithet using isana tori, which literally meant whaling but figuratively could refer to the sea.
The above probably aligns more with what the OP was expecting, and shows how popular whaling was.
The part about kujira matches what Atamasama said, but gives a bit more info. I wish it also told when it started being used over isa(nu).
To expand on this a little bit, the left side of the character is called the radical (as I was taught) and it gives a general sense of the area of meaning; in this case, fish. The right side of the character (again, as I was taught) usually has more to do with pronunciation than with meaning. However, since as Sage Rat says, the character was adopted only in written form from Chinese and the Japanese oral word grafted onto it, the search for the pronunciation of “kujira” in Japanese would be fruitless.
Incidentally, according to my kanji dictionary, the (adopted and probably changed over time) Chinese pronunciation of the character for Whale is Gei, which is similar to the (similarly adopted and changed over time) Chinese pronunciation of the right side of the character as Kei, which currently corresponds to the meaning “ten quadrillion.”
I trust this is all suitably obscure.
Thanks, everyone.
Just to clarify, that’s the Chinese-derived pronunciation used in Japanese, as in ‘whaling’ 捕鯨, catch-whale literally in Chinese, pronounced hogei in Japanese, bǔjīng in Mandarin, pogyeong in Korean, same word in all three as is pretty common.
As several others have said, Chinese characters don’t tell you anything about the etymology of Japanese indigenous words which were morphed onto them.