I saw 火信 tattooed on a gentleman's neck. I looked up the words, but couldn't make sense of their combination...
What does it mean?
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Update: It seems that it is most likely that the person who created it was intending a Chinese word. However, since the kanji is valid Japanese kanji, and since this is a forum about learning Japanese, I'm going to form my answer based on the perspective of "what if they meant it to be Japanese?"
It is not a standard Japanese compound. However, that doesn't mean it therefore can not be Japanese, and must be Chinese, or a mistake by the person who wrote it.
If the two kanji were incorrect in the way they were drawn, or had extra strokes only found in Chinese, then it could not be considered Japanese in any way. However, both those kanji are standard Joyo kanji.
Japanese kanji can be combined to make new words, and is done frequently as a part of the evolution of the language. People who are native or fluent enough have the freedom to create new words, and when done right, people who have never seen it before can catch the meaning. We do this all the time in English with prefixes, suffixes, portmanteau, and other ways.
Or, in this case, one could import the word from its Chinese origins, maybe because they thought it was a cool concept that the Japanese language didn't have yet.
So, that said, assuming someone wants the word to be Japanese, forget the fact that this isn't in the dictionary. It's a new word, but still can be a Japanese word nonetheless.
The kanji mean "fire" and "belief", respectively, and from there we can extrapolate that it means something like "burning faith". We're factoring a bit of poetic license in, because of the context of a tattoo, where that kind of sentiment is expressed.
After testing it on some Japanese friends, it would definitely be read かしん, and half I tested on guessed what I think is the intended meaning of "burning faith". The others guessed "belief in fire". No one guessed "firewire", or anything to do with the "transmission" definition of 信.
But that's what happens when you make up new words. You don't necessarily get perfect comprehension right off the bat. So you have to say "well, to me it means...". Which is also a pretty common conversation people have when they are talking about their tattoos.
So, in short, I think the consensus is that this is probably a Chinese word that happens to use kanji that are commonly used in Japanese. However, if we wanted it to be a Japanese word, that would be perfectly valid.
Cultural observation: I believe that if the gentleman with the tattoo was visibly identifiable as non-Japanese, it's very likely a lot of Japanese would probably tell him his tattoo was not a real word, assuming he made a mistake. If the gentleman was Japanese, then it would be assumed he was at play with the language.
It's an extension of the unfortunate belief held by many Japanese that the Japanese language is particularly inaccessible to non-Japanese people.
I don't find them combined as 1 word in Japanese dictionaries I referenced, but if you google them you get many hits on Chinese sites. So I assume it means "burning faith", but it's Chinese, not Japanese.
It is not Chinese either. 火信 doesn't make any sense in Chinese. But we Chinese have a phrase 火蛇吐信.
But 火信 won't mean "Fire tongue", 信 will only mean "tongue" in limited cases such as 火蛇吐信.
So 火信 doesn't make any sense in Chinese. You guys can discussion the other possibilities.
Firewire (火-fire, 信-communication/transmission)??? Nothing in Japanese anyway. Not to say it doesn't mean something in Chinese. Maybe 当て字 for his name or something. But chances are it's just one of those tattoos that's supposed to look cool, but doesn't mean a dang thing and ends up looking stupid.
May be, you could be reading it reverse.
That could be 信火 from Buddish word 信火行煙, which means
Some years ago there used to be a web site called "hanzismatter.com". (Hanzi is Chinese for kanji.)
This site was a blog where there were regular postings of photographs of people's really dumb Chinese character tattoos.
For instance, one guy wanted to have the word "tank" (probably a military man). But the kanji that were used actually gave rise to a meaning more like "septic tank", not the armored vehicle.
Another guy ended up with something that meant "fried chicken".
Tattoos are often wrong. Some are plain incorrect, like missing or incorrect strokes, or outright mirror-image reversals. Some have silly meanings pulled from some literal translation, or a juxtaposition of characters that individually mean something, but accidentally create a bad compound.
So the point is, there isn't always a meaning.
This person probably just pulled two nice characters out of the tattoo artist's "catalog", which probably had some possibly incorrect meanings next to them. That person liked the meanings, and was probably given a "2 for 1" special, so had two characters "done".
Oh, look that blog is up again, on blogspot:
Wow, this could be the world's first post of a blogspot link that isn't spam. :)