A person who spends an inappropriate amount of time and effort flattering or pander to their in-laws in order to gain their approval.
In its worst form, an in-law fluffer seeks the attention of their in-laws at the expense of their partner.
Implies that the person is fluffing their in-laws; sexually pleasuring them to keep them aroused.
Example 1. Woman- I was talking to your dad today and thought how much he would enjoy the game on sat night. I told him he could have our tickets.
Man - What? You have become a real in-law fluffer since that bad holiday last spring.
Example 2. Why did you get flowers for my sister? Quit fluffing her, she already thinks you are a spaz.