1
general::
Also, log on. Enter into a computer the information needed to begin a session, as in I logged in at two o'clock, or There's no record of your logging on today. These expressions refer especially to large systems shared by numerous individuals, who need to enter a username or password before executing a program. The antonyms are log off and log out, meaning "to end a computer session." All these expressions derive from the use of log in the nautical sense of entering information about a ship in a journal called a log book. [c. 1960]
American Heritage Idioms