Probably nothing to do with the term, but interesting just the same.
Don't you just love questions about things you take for granted, but turn out to be more comlicated when you look into them - thanks Sylvia.
Jim Carroll

From;
The Knot Tied, Marriage ceremonies of all nations, collected and arranged by William Tegg 1877

"HAND-FASTING" IN SCOTLAND.
Hand-fasting, or hand-fisting, was a form of marriage for a limited period, which prevailed in some parts of Scotland up to the early part of the last century. It was most common in Eskdale, at the confluence of the Black and White Esks. One of the contributors to Sir John Sinclair's Statistical Account of Scotland, writing in 1794, says:—At an annual fair, held time out of mind, but now entirely laid aside, it was the custom for the unmarried persons of both sexes to choose a companion according to their liking, with whom they are to live till that time next year. This was called hand-fasting, or hand in fist.    If they were pleased with each other at that time, they then continued together for life; if not, they separated, and were free to make another choice as at the first. The fruits of the connexion, if there were any, was always attached to the disaffected person.
In later times, when this part of the country belonged to the Abbacy of Melrose, a priest, to whom they gave the name of "Book i' Bosom"— either because he carried in his bosom a bible, or perhaps a register of the marriages—came from time to time to confirm the marriages.    Eskdale is only a short distance from the Roman encampment of Castle-o'er.   May not the fair have been first instituted when the Romans resided there ? and may not the "hand-fasting" have taken its rise from their manner of celebrating marriage, ex usu, by which, if a woman, with the consent of her parents or guardians, lived for a year with a man, without being absent three   nights, she became his wife? Perhaps, when Christianity was introduced, this form of marriage may have been looked upon as imperfect without confirmation by a priest, and therefore one may have been sent from time to time for this purpose.
Instances of Hand-fasting as far north as the Hebrides have been narrated by travellers in that then remote part of Scotland.