cistern vs nest egg

cistern

noun
  • A cisterna. 

  • The vessel surrounding the condenser in a steam engine. 

  • In a flush toilet, the container in which the water used for flushing is held; a toilet tank. 

  • A reservoir or tank for holding water, especially for catching and holding rainwater for later use. 

nest egg

noun
  • An egg squash. 

  • A natural or artificial egg placed in a bird's nest, to encourage the bird to lay its own eggs there. 

  • A saving; a reserve of money. 

How often have the words cistern and nest egg occurred in a corpus of books? (source: Google Ngram Viewer )