Meaning – to get married. For example, "John ties the knot with Mary."
There is a suggestion that this expression derives from the nets of knotted string which supported beds prior to the introduction of metal-sprung bedframes. The theory goes that, in order to make a marriage bed, you needed to 'tie the knot'. Like many such folk-etymological explanations, there's not a shred of evidence to support this idea.
It isn't clear whether this expression derives from an actual knot used in marriage ceremonies or whether the knot is merely symbolic of a lasting unity. Knots have a place in the folklore of many cultures and usually symbolize unbreakable pledges. Actual knots have certainly been used in marriage ceremonies for some time and the tradition of trying the wrists of the bride and groom with twine continues today in marriages in the use of sashes which are placed over the principal's wrists. The word 'knot', although not in the phrase 'tie the knot', has been associated with marriage since at least the 13th century. The Legend of St. Katherine, circa 1225 used the Middle English 'cnotte', i.e. 'knot', to mean 'the tie or bond of wedlock; the marriage or wedding knot':
"Swa ye cnotte is icnut bituhhen unc tweien."
E. and M. A. Radford's The Encyclopedia of Superstitions has it that:
"In the seventeenth century, one or two of the bride-favours were always blue. These were knots of coloured ribbons loosely stitched on to the wedding gown, which were plucked off by the guests at the wedding feast, and worn as luck-bringers in the young men's hats."
The expression was recorded in 1717 by the English poet and diplomat, Matthew Prior. In his humourous poem, Alma; or, The Progress of the Mind he includes:
"So to the priest their case they tell: He ties the knot."
Francis Grose, in his 1811 edition of The Dictionary of the Vulgar Tongue listed the 'knot tied with the tongue' with specific reference to marriage.
"He has tied a knot with his tongue, that he cannot untie with his teeth: i.e. he is married."
The Jewish tradition, like many others, also has a long history of the use of knots in the marriage ceremony. The Wilmingtonian And Delaware Advertiser, January 1826, reported this item under the banner of 'Miss Rothchild's Marriage':
"At an early hour on Monday morning, Stomford Hill, the country residence of N. M. Rothschild, Esq. was in a great state of bustle, and most of its inhabitants were on qui vive, waiting the approaching hour when Hymen was to tie the knot of a son and daughter of the house ot Judah."