What is the origin of 'sleep tight'?
Read moreOne of the mysteries of the English language finally explained.
(Originally) †the extent to which a person shows coldness, as one of the characteristics of phlegm (obsolete); (in later use) the extent to which anything is cold; specifically the number of degrees below the freezing point of water; frequently in plural.
Late 16th century; earliest use found in Richard Carew (1555–1620), antiquary and poet.