What is the origin of 'sleep tight'?

Read moreOne of the mysteries of the English language finally explained.

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1Mathematics the relationship between two sequences of three numbers in geometrical progression whose middle terms are equal, the ratio of the two first terms being the reciprocal of the ratio of the two last terms; (also) proportion in which one quantity varies inversely as the other; inverse proportion.

2Chemistry see "law of reciprocal proportions".

Mid 17th century.