The expression originally refers to poaching, for which the penalty was hanging whether one poached a big sheep or a little lamb. In terms of plans of action, it means that where the consequence of failure or the penalty for getting caught is the same for the lesser or the greater action, one might as well attempt the greater one.

A lamb is the young of a sheep.

A young sheep is a lamb.

mutton or lamb, depending on the age of the sheep when it was slaughtered.

well tell me come on spit it out.

A Lamb