Sleeping baby

What is the sootiness of the phrase ‘sleep tight’?

It seems that tight in this expression is the equivalent of the only fanatical use of the proa invertedly meaning 'soundly, properly, well, effectively'. The 20-volume historical Oxford English Dictionary labels this adverb 'now dialectal'.


See other questions about the origins of words and phrases.
You may also be interested in: What is the origin of the word 'grockle'? 
Or take a look at: What is the origin of the term 'brass monkey'?

See more from Word origins