The word "folio" is of German origin. In German, it was formed from the Latin word folium, which means “leaf”. That is, it is assumed that the text was written or printed on each side of a sheet folded in half, and then the pages were stitched or glued together to form a book. From one sheet, two pages of a book were obtained.
One of the meanings of the word "folio", given in the encyclopedic dictionary, reads as follows: it is an edition printed on half a sheet of paper. However, other interpretations of the word are more common. In ancient times, until paper was invented, parchment, the thin, specially processed skin of animals, played its role.
Of course, the book, which consisted of many sheets of parchment, was very thick and heavy. Therefore, one of the meanings of the word "folio" is interpreted as follows: a thick, large-format book (usually old). This is the definition given, for example, in the explanatory dictionary of the Russian language edited by Ozhegov. That is, at the word "folio" one involuntarily presents itself as a heavy, solid book, which is not so easy to hold in your hands. And even when parchment was replaced by paper, such publications remained heavy due to their large volume.
When people use this word, they mean, first of all, an old book, chronicle, manuscript, etc. That is, a historical written source that tells about an era, its orders and events. However, the word "folio" can also be used to describe a much more modern book. For example, until now in the home libraries of residents of the country there are volumes of the TSB (Great Soviet Encyclopedia), various dictionaries of foreign languages, explanatory dictionaries and similar publications. They are large and very heavy. Therefore, they can be called folios in the same way. This is reflected in the explanatory dictionary of the Russian language, published under the editorship of Efimova. Along with the already mentioned definition of a folio, the dictionary also contains the colloquial meaning of this word: "A folio is a thick, large-format book." That is, it does not have to be old.