impair vs spin down

impair

verb
  • To weaken; to affect negatively; to have a diminishing effect on. 

spin down

verb
  • To diminish in energy; to slow down or peter out; to be gradually canceled or ended. 

  • To reduce its speed of rotation. 

  • To reduce its spinning speed from that required for reading and writing. 

How often have the words impair and spin down occurred in a corpus of books? (source: Google Ngram Viewer )