To "tie the knot" is to get married to a mate or for a cleric or other official to perform a wedding ceremony or civil union.
The term comes from many different cultural marriage ceremonies where a couple either ties a knot in a ribbon or decorative rope or one is tied around their wrists by the person performing the ceremony. This signifies the union of two separate entities into one spiritually combined one. Some of the knot tying ceremonies include a short statement by both parties similar to wedding vows.
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