[bye-bye] or by-by interj [baby-talk redupl. of goodbye] (ca. 1736)--used to express farewell
[2]bye-bye or by-by adv (1917): out esp. for a walk or ride--used with the verb go [3]bye-bye or by-by n (1867): bed, sleep "lie down ... and go to ~ --Rudyard Kipling" [4]bye-bye or by-by adv (1920): to bed or sleep--used with the verb go