What is the origin of 'sleep tight'?
Read moreOne of the mysteries of the English language finally explained.
Definition of day-day in US English:
day-day
exclamation
informal‘Goodbye’. Also as noun in "to shake a (also one's) day-day" and variants: to wave goodbye.
Origin
Late 17th century; earliest use found in Elkanah Settle (1648–1724), playwright. Reduplication of day.
