day-day
/ˌdeɪˈdeɪ/informal
exclamation
‘Goodbye’. Also as noun in to shake a (also one's) day-day and variants: to wave goodbye.
- Frequently in representations of the speech of small children, or that used by adults in speaking to small children..
Origin
Late 17th century; earliest use found in Elkanah Settle (1648–1724), playwright. Reduplication of day.