I vaguely remember telling someone on a technical mailing list, when the conversation had turned to food, that tofu is not rotten soybeans.
He had learned from someone that the "fu" in "tofu" meant "rotten".
I am not always on the top of everything. That's all I can say.
I can't find the post today, but let me try to set the record straight.
The "to" in both "tofu" and "natto" is "bean". Without other qualification, it often (but not always) means soy bean.
(It is actually read, 「とう」、 which would have a literal Romanization as "tou", but lengthened vowels are often ignored in Romaji. Oh. In Japan, Latinization is Romanization, or Romaji.)
The "tou" in natto and tofu is also read "mame" (mah-meh, sort of), and it means "bean". (Sometimes, it is used non-literally to mean "clever", as well. If you are a mame, if you are a bean, you are clever. I like that.)
Indeed, the "fu" in "tofu" means, erm, well, "rot", when it is read "kusaru". But it really means "aged", as in aged cheese. (Think, "funky". Have you ever heard of funky cheese? I'm not mentioning beer, okay?)
That's one reason why they used to call tofu "soy cheese". (Some people do still call it soy cheese.)
But it would be more appropriate to think of cottage cheese than, say, bleucheese. (What happened to the wikipedia article on bleucheese? They misspell it and don't even mention the derivation. Re-writing history? Evidence that the crowd can lose touch?)
Tofu is not really made by aging any more, except for the more expensive kinds. The soy milk is curded in a method that is similar to the methods used in curdling milk to make cottage cheese. Doesn't even take more than a day to make most of the tofu you buy in the stores.
(You can make cottage cheese with vinegar or lemon juice instead of rennet. In a similar way, you can make tofu with nigari, which is often a magnesium salt.)
The "natsu" (納) in "natto" (納豆) means to store away, or to put something where it belongs. It is also pronounced, "osameru", which is another word used to describe paying taxes: "Zeikin wo osameru." (税金を納める。) or "nouzei" (納税). (Yeah, in nouzei, the same character is read "nou" instead of "natsu". Nothing unusual about that with Kanji characters.)
But natto is fermented soybeans. Last time I looked, Wikipedia had a pretty good explanation of why.
In brief, a long time ago, maybe in China, according to the traditions, some high-muckey-muck in the army ordered a lot of soybeans, not knowing what to do with it. But when no one in camp knew what to do with it at all, they just buried it away in disgust.
Some time later, when the whole camp was about to perish for want of food, someone noticed the dogs happily digging into the buried trash. They spied on the dogs and saw that they were into the buried soybeans. And they all decided, if the dogs are eating it, maybe we can try it, so they dug up the buried, and now fermented, soybean, and tried it, and lived. And had strange tastes in food when they got home.
(There are many versions of this story. Don't take it too literally.)
So, natto could be called rotten soybean. Fermented soybean sounds better, and is more accurate.
I like natto. When I couldn't eat chocolate, I found natto made (for me) a good substitute for chocolate. (I now eat pure cacao mass fairly regularly. Good stuff, although you don't want to eat a lot of it at once.)
Modern natto doesn't even taste all that strongly of ammonia. And it has a lot of the good stuff that you would only otherwise find in Japan in expensive meats -- amino acids, B vitamin predecessors, protein, etc.
There are valid reasons for not being able to stomach either natto or tofu. For instance, allergies to soybean do exist.
But, if someone tells you they won't eat rotten soybean, and that's why they won't try tofu, maybe they've been confused by dictionary definitions again. Natto is probably what they are thinking they should avoid.