Tie the knot
Definition 1 of 1
Pro Tip 1/3Metaphorical Origin
Originally, 'tie the knot' came from the ancient practice of tying a couple's hands together in marriage. 
They will tie the knot next summer, symbolizing their unbreakable bond.
Pro Tip 2/3Emotional Joy
Using 'tie the knot' suggests happiness and celebration associated with marriage. 
My sister and her fiancé are ecstatic to tie the knot this fall.
Pro Tip 3/3Informal Use
'Tie the knot' is informal and often used in casual conversation, rather than official documents. 
Did you hear? Mike and Jessica decided to tie the knot!