The term "free muff" can be described as the act of lying in bed while a woman's intimate area "muff" rests on her partner's leg in a relaxed and casual manner.
An example of "free muff" might be after a disagreement, when one partner goes to get a drink. On returning to the room, after a moment of listening outside, they enter, place the drink on the side, and get into bed. The other partner then casually drapes their leg over theirs, and they notice a sensation of moisture. This leads to the realization that their partner's intimate area "free muff" is in direct contact with their leg in a relaxed and unassuming manner.