diary (n.) Look up diary at Dictionary.com
1580s, from Latin diarium "daily allowance," later "a journal," neuter of diarius "daily," from dies "day" (from PIE root *dyeu- "to shine," in derivatives "sky, heaven, god"); also see -ary. Earliest sense was a daily record of events; sense of the book in which such are written is said to be first attested in Ben Jonson's "Volpone" (1605).