this link, it is a dialect version of "novelty". I think it is used as a noun. I think the meaning of the whole thing is "He believed that he did not just have one mistress, he had more than one. He had heard that the man liked novelty". Ignoring the morals or ethics of the whole thing, having one mistress presumably becomes rather boring. Having more than one mistress keeps the interest and the excitement going and presumably each time he gets a new mistress, he experiences new things and new feelings. In this way, he fulfils his liking for novelty.
You have to bear in mind that you are reading pieces from the 1700s. Spellings have changed a lot and become more standardised. "Shew" would have been a perfectly acceptable and understandable word in those days. See here.