There are a variety of possible sources for the origin of this saying. One suggests that brides
in the eighteenth century wore knots of coloured ribbons attached to their wedding gowns,
while another states that the term derives from the Celtic hand-fasting ritual in which the man
and wife's hands were bound and tied to signify their union and the completion of the wedding ceremony. A more common explanation, however, stems from the fact that at one time beds
were made from planks of wood held together by rope. On the wedding night, the bed was
thought to require additional strength and stability from an extra knotted rope as the couple consummated their vows.

Similiar to this is the phrase 'to sleep tight', the origin of which is thought to derive from the
frames of such beds being sprung with ropes. If the ropes went slack, it resulted in an uncomfortable night's sleep. The ropes had to be tight so that a person could 'sleep tight'
(ie well).