To move about restlessly while in bed. If this alliterative expression were older, it would be redundant, since to toss once also meant to turn over and over. However, it did not surface until the late 1800s, and it is always used for this behavior while one is in bed. J. R. R. Tolkien used it in The Fellowship of the Ring (1954), “He lay tossing and turning and listening fearfully to the stealthy night-noises.”