The real evils, indeed, of Emma's situation were the power of having rather too much her own way, and a disposition to think a little too well of herself; these were the disadvantages which threatened alloy to her many enjoyments. The danger, however, was at present so unperceived, that they did not by any means rank as misfortunes with her.
What does Jane Austen mean when she writes "which threatened alloy to her many enjoyments". I get the general meaning, but I would like to know the exact meaning of "alloy", is it a noun or verb? How does it function in the sentence?