Apologies in advance, as we are going to be a wee bit repetitive here. Repetitive because Japanese has this amazingly amazing capacity to form words through the simple process of repetition, or what linguists call reduplication.
The Japanese term for this craft is 畳語 (jōgo), a compound of the characters for 畳む (tatamu, fold) and 語 (go, word). The idea is that there are two layers of the same piece of structure folded over one another, just like the straw layers of a tatami mat. So, for the sake of simplicity let’s call them tatami words — and unfold them a little bit.
Perhaps the most frequent — and definitely best-known — type of tatami words are onomatopoetic expressions. It’s きらきら (kirakira) for things that twinkle, かりかり (karikari) for things that crunch, ぺらぺら (perapera) for speaking a foreign tongue with ease, and べらべら (berabera) for doing the same with one’s own tongue. These are just a few of an almost uncountable number of such expressions. In fact, there are so many that one sometimes wonders whether there is any repeat combination of two kana left that does not claim some onomatopoetic meaning.
Imitating sounds is also a factor that may produce tatami words in child language. Probably best known among these are わんわん (wanwan) for dog, にゃーにゃー (nyānyā) for cat, and ぶーぶー (būbū) for car. However, reduplication in the speech of children may also occur without any onomatopoetic science involved. Some examples of this type are おめめ (o-meme, eyes), おちんちん (o-chinchin, willy) and おてて (hands).
In some cases, saying something twice may serve the very straightforward purpose of emphasizing there is more than one of that something. Though Japanese does not normally mark words for plural, there are a few cases where this “more is more” principle applies. Often using the repetition mark 々, these include 人々 (hitobito, people), 国々 (kuniguni, countries) and 日々 (hibi, days).
Related to plural and yet somewhat different, there are a lot of cases where tatami words express heterogeneity or individuality. Examples that come to mind here are 色々 (iroiro), 様々 (samazama), and まちまち (machimachi), all meaning “various,” or “a large variety of.” The individuality of people in a group comes across very well in terms such as 個人個人 (kojinkojin), a reduplication of the noun “individual,” and 一人一人 (hitorihitori), repeating that “each single person” is concerned. Note that this latter term is commonly written as a kanji-kana blend (一人ひとり), as if to visually emphasize that no two people are the same.
This brings us to yet another important function of repeats, which is to express force or intensity. If you do your work ガンガン (gangan), you are toiling away like a madman, and if you meet someone you haven’t seen for ages, it’s 久々 (hisabisa). Something that happens in the twinkling of an eye can be labeled as 見る見る (mirumiru), while things that seem to go on forever are described as 延々 (en’en).
Many tatami words carry a sense of expressiveness that goes beyond mere intensity. A doubled 寒い (samui, cold), as in 寒々 (samuzamu), conjures up images of wintry bleakness not exclusively related to cold. On the other end of the temperature spectrum we have 熱々 (atsuatsu), which may be used to refer to something scalding hot, but can also be said about people passionately in love. A similar notion, albeit with a more lighthearted undertone, is expressed by ラブラブ (raburabu) from the English “love,” illustrating the fact that repetition does not even shy away from loanwords.
And neither is it restricted to the word level. Take for example the title of Hirokazu Koreeda’s 2008 movie “歩いても歩いても” (“Aruite mo Aruite mo”) — itself a quote from Ayumi Ishida’s iconic 1968 hit single “Blue Light Yokohama” — where the conjunctive form of the verb aruku (walk) and the particle mo are doubled to convey a sense of indefinite repetition — a meaning somewhat reflected by the English translation of the title, “Still Walking.” Add a negative verb form to this pattern and you can express impossibility with a touch of despair, as in 読んでも読んでもわからない (Yonde mo yonde mo wakaranai, “No matter how often I read this, I just can’t understand it”) or 待っても待っても来ない (Matte mo matte mo konai, “I wait and wait, but so-and-so just doesn’t show”).
Speaking of so-and-sos, tatami words may also be used as placeholders, in a very similar way to how the English example sentence above does. Expressions like 何々 (naninani) and 誰々 (daredare) are quite helpful when you need to refer to an unspecified thing or person, respectively. And we mustn’t forget 云々 (unnun), which comes in most handy when you would like to indicate that you could go on about this for ages but think you’ve said enough and would like to leave it at that. Best English translation: and so on.