interj. /buy"buy"/ ; n. , adv. /buy"buy'/ , interj.
1. Informal. good-bye.
n.
2. Baby Talk. sleep.
adv.
3. go bye-bye , Baby Talk.
a. to leave; depart; go out.
b. to go to sleep; go to bed.
[ 1700-10; appar. orig. nursery phrase used to lull a child to sleep, later construed as reduplicative form of BY 2 , short for GOOD-BYE ]