Jessaboo - 'lights' is the Old English name for the lungs, and refers to 'the light (in weight) organ'. The lungs are also the softest internal organ - they're sort of lightweight and squishy soft. (The word 'offal' is a general term for edible internal organs removed from an animal after slaughtering, including the heart, liver, kidneys and intestines).
This refers to the character of a person, and how 'soft' they are i.e, a 'soft touch' will always lend you a pound. 'Soft as Lights' originally comes from the farming communities that slaughtered their own cattle. They were very aware of how soft the intestines (lights) of the animals are. Hence 'Soft as Lights' BazLG