Where does the expression “Dilly Dally” come from and What does dillydally mean?

Who first used the expression Dilly Dally, or when, is not known; probably some harassed mother back in the sixteenth century, annoyed with the dalliance of a loitering son, or husband, made dally more emphatic by duplicating it with dilly.

At least, the dilly prefix has no separate meaning of its own, any more than shiny has in the analogous term, shilly-shally.