1581, from L. diarium, “daily allowance,” later “a journal,” neut. of diarius “daily,” from dies “day.” Earliest sense was a daily record of events; sense of the book in which such are written is first attested in Ben Jonson’s “Volpone” (1605).

Edit:

The 9th century scholar Li Ao, for example, kept a diary of his journey through southern China.

Sales of “page a day” diaries go back hundreds of years (Letts, for example, is over 200 years old).

The modern Western stereotype of a diary is a record kept by teenage girls, usually concerning such matters as school, parents, and immature attempts at romantic liaisons.

For many years, the only inexpensive diaries on the market featured pastel covers with naively romantic cover art and flimsy locks and keys, thus perpetuating this illusion. However, this type of diary and the accompanying cultural associations did not exist until the 1940s.

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