A rolling ball of cock. Used as an insult. When someone is beyond just a cock, they are so much cock that it can't stay in one place.
To use the word and the idea best, you need to just yell the word into the face of the tumblecock.
"Bob is a tumblecock!"
"Yea he is, watch this" *walks over to Bob and leans into his face* "TUMBLECOCK!"