grass vs stool pigeon

grass

noun
  • An informer, police informer; one who betrays a group (of criminals, etc) to the authorities. 

  • Asparagus; "sparrowgrass". 

  • Marijuana. 

  • Sharp, closely spaced discontinuities in the trace of a cathode-ray tube, produced by random interference. 

  • The season of fresh grass; spring or summer. 

  • The surface of a mine. 

  • A lawn. 

  • Various plants not in family Poaceae that resemble grasses. 

  • Noise on an A-scope or similar type of radar display. 

  • Any plant of the family Poaceae, characterized by leaves that arise from nodes in the stem and leaf bases that wrap around the stem, especially those grown as ground cover rather than for grain. 

verb
  • To bring to the grass or ground; to land. 

  • To feed with grass. 

  • To act as a grass or informer, to betray; to report on (criminals etc) to the authorities. 

  • To lay out on the grass; to knock down (an opponent etc.). 

  • To cover with grass or with turf. 

  • To expose, as flax, on the grass for bleaching, etc. 

stool pigeon

noun
  • An informer to the authorities, especially one who gives up criminal associates. 

  • A pigeon tied to a stool by a piece of twine, used as a decoy to entice free-flying birds to the ground to be shot or trapped. 

How often have the words grass and stool pigeon occurred in a corpus of books? (source: Google Ngram Viewer )