James C. Russel in The Germanization Of Early Medieval Christianity claims its origin is the Germanic custom of addressing ones superior, lord, liege this way, to kneel and fold hands to show submission. Oath of fealty given as kneeling and folding hands, wit the liege putting his hands around the folded hand.

Russels book is a very... unusual one, as it tends to claim far more Paganic/Germanic influence in Christianity than the mainstream, so to speak, so cannot vouch it is right, but at least it is an interesting hypothesis backed by a source.

There is an even more interesting hypothesis that this sign of submission comes from prisoners having their hands bounded together, but I cannot find any reputable source for that, likely just speculation.