spotted dick and spotted dog are both popular British suet puddings which are ‘spotted’ with raisins. Strictly speaking, spotted dick is of the ‘roly poly’ type (see roly poly) with raisins and sugar spread on a flat sheet which is rolled up; and spotted dog is a plain cylinder with raisins or currants and sugar mixed with the suet paste, so that it has visible spots on the outside. Both, correctly, should be boiled in a cloth, but are often now baked.
See soda bread for another meaning of spotted dick.