What is the origin of 'sleep tight'?

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

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An expression involving derivatives of a function y with respect to a variable x which has the same form (up to multiplication by a factor) when expressed with derivatives of x considered as a function of y. Now historical.

Mid 19th century (in an earlier sense). From classical Latin reciprocant-, reciprocāns, present participle of reciprocāre.