Tom Tiddler's ground a place where money or profit is readily made.

☞ Tom Tiddler's ground was the name of a children's game in which one of the players, named Tom Tiddler, marked out their territory by drawing a line on the ground. The other players ran over this line calling out 'We're on Tom Tiddler's ground, picking up gold and silver'. They were then chased by Tom Tiddler and the first (or, sometimes, the last) to be caught took his or her place.