Many of the Ringbearers (particularly those who carried it longer) used the word "precious" to describe it. Isildur described it as:
"But for my part I will risk no hurt to this thing: of all the works of Sauron the only fair. It is precious to me, though I buy it with great pain."
Bilbo himself used it at least once, when arguing with Gandalf about leaving it behind when he was leaving Bag End (this exchange and the lines were different in the movie, but the word is still there):
"Well, if you want my ring yourself, say so!" cried Bilbo. "But you won’t get it. I won’t give my precious away, I tell you."
And Gollum used it incredibly frequently, almost as if it was the ring's name. It was even his last words:
"Precious, precious, precious!" Gollum cried. "My Precious! O my Precious!"
So my question: Why "Precious"? It wasn't part of the inscription. It wasn't part of the poem that included the inscription. Was it somehow part of Sauron's crafting spell? Did Tolkien ever indicate why the Ringbearers would use that word in particular, other than that it sounded good?