The phrase "goose that laid the golden egg" refers to a source of great wealth or success that should be maintained and nurtured carefully. There exist several synonyms that describe the same concept. One such synonym is "cash cow", which is used to describe a reliable and profitable business venture. "Golden opportunity" is another synonym that highlights a chance for great success or fortune. Similarly, "treasure trove" refers to a source of valuable items or information. "Pot of gold" is another commonly used phrase that signifies a valuable prize or opportunity. These synonyms can be used interchangeably to convey the same idea of a lucrative asset that should be preserved with care.