What's the meaning of "Sleep Tight"

Today's topic I'm going to talk about the Idiom "Sleep Tight" and its origin. This 19th century expression isn't, as is often wrongly claimed, a reference to the tightness of the strings used to support mattresses. 'Tight' just means 'soundly/properly' and 'sleep tight' just means 'sleep soundly'. The word was probably chosen because of its rhyme with night, so people wished other 'good night, sleep tight'.

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I hope this could be helpful! Don't forget to share the English tips for your friends around the world! 

Source: The Phrase Finder