A common symbol in modern weddings it the image of knot. The phrase "tie the knot" as a euphemism for marriage that is also commonly recognized. Where does this originate from?
The metaphor of a knot is one of binding, as two people are bound together in marriage.
Shakespeare uses the metaphor, but not the exact phrase:
But it's earlier still, in The Legend of Saint Katherine c 1225:
Or as a rough stab at a translation into modern English:
It may relate to knot-tying as an actual part of wedding ceremonies (and sometimes betrothal ceremonies), as is found in rituals from throughout the world from ancient times until the present day.
Or it may just be a metaphor applied in the phrase alone.
Being so old, it's probably impossible to tell which.
Either way, with knot being in English for so long as a symbol of marriage, and tying being how one forms them, "tie the knot" was pretty inevitable from that starting point.
*Some notes on translation, since they might be of interest in their own way:
I'm tempted to have it "...us twain" rather than "...us two" but while twain is Modern English, it's not common in Contemporary Modern English. It would be a closer translation.
A little is lost in translating wit to we and unc to us as Modern English doesn't have pronouns that specifically refer to two people. "We two" and "us two" would be more accurate but more clumsy.
Knitted would be a rare choice for a knot today—not unheard of, but rarer than once was the case (e.g. see the Shakespeare quote as an example in Early Modern English). Tied or fastened would perhaps be a better translation for that reason, but it would introduce a repetition that scanned a bit silly to my eye.
I'm not happy about the translation of "in an" as "together", there's a bit of guesswork there. Often it's the little prepositions and conjunctions that are trickiest for a non-expert like me, as they're used quite differently in Modern English. I'd be open to other suggestions.
Similarly, swa is the root of the Modern English so but also used as we would use that, then (consider how Irish English still uses so where other dialects would only use then) and as. This mean that the form "as ... as ..." (e.g. "as white as snow") was more flexible in Old and Middle English than today. Translating sadly loses the poetry of the repetition.
To tie the knot is a remnant of the ancient church's tradition to take the couple's hands and tie them with a piece of fabric during the wedding in the Church. In the West, this tradition has fallen in disuse, but in an unchanged form, this is how it is still done in the Orthodox Church.