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1640s, from French liaison "a union, a binding together" (13c.), from Late Latin ligationem (nominative ligatio) "a binding," from past participle stem of Latin ligare "to bind" (see ligament). Originally a cookery term for a thickening agent for sauces. Sense of "intimate relations" is from 1806. Military sense of "cooperation between branches, allies, etc." is from 1816. The noun meaning "one who is concerned with liaison of units, etc." is short for liaison officer.