(1) A wet sleeping bag. If this happens because of a hole or faulty seam in your tent, or because so much rain fell that some crept in from below, fine. But if this is because of something you did in your sleep, then we don't want to share, especially not your tent. We'll carry our own, thanks.
(2) An extensive, often peat-rich bogginess covering large areas of gently undulating uplands in cool yet seriously wet climates. Moisture from rain is so prevalent, and due to the cool climate so persistent, that it collects in high hollows and even on the gently sloping hillsides, and remains in place long enough to nurture the types of plants that flock to bogs for excitement and even extended stays.
Stays so extended that the plants live out their lives, die, fall over, and after many years of being dead become peat. Layers of peat. Layers of peat that cover the landscape like an enormous blanket going on for miles and miles over hill and dale yet never relinquishing its fundamentally sodden and depressing nature.
(3) In addition to "fen", "blanket mire" is another term. It's a British phrase, and they know about such things.
Another alternate name, in case you can't stick with what you've got already, is "mire".
Mire is a nice word. It can't be mistaken for anything else. And there's only the one syllable to remember as you sink into it.