(v.) to decorate with evergreen branches
According to the English Dialect Dictionary, to boun is “to decorate with evergreens at Christmas”.
These days, those evergreens are more likely to be paper or plastic than they are actual evergreen branches and boughs of holly, but nevertheless bouning is clearly a useful word when Christmas rolls around—but originally, it was a lot more useful than that.
The Oxford English Dictionary traces the earliest record of boun back to the late fourteenth century, when it was originally used in a much broader sense to mean simply ‘to make ready’. Earlier still, boun was an adjective in Middle English meaning ‘fully prepared’; the quippy turn of phrase ready and boun, or just ready-boun, was essentially a mediaeval equivalent of the ready and willing or ready and able we might use today.
But as time went by, confusion between the adjective boun and the verb boun—or, more specifically, its derivative bouned, meaning ‘readied’—led to the adjective gaining an extra letter: by the early fifteenth century, bound with a D was being used to mean ‘prepared’ or ‘ready to go’ (which is, incidentally, the same bound that appears in in stock phrases like homeward bound and outward bound).
Likewise, around the same time the verb boun began to change. It came to be used in a handful of more specific senses, including ‘to dress’, and it’s perhaps from there—presumably in the sense of “dressing” or “attiring” your home in readiness for Christmas—that the festive use of the word developed in the late 1700s.