relationship vs side piece

relationship

noun
  • A romantic or sexual involvement. 

  • The level or degree of affinity between keys, chords and tones. 

  • The links between the x-values and y-values of ordered pairs of numbers especially coordinates. 

  • Connection or association; the condition of being related. 

  • Kinship; being related by blood or marriage. 

  • A way in which two or more people behave and are involved with each other 

side piece

noun
  • A mistress; a woman with whom a man carries on a physical affair (not emotional affair) without having the intention of forming a serious relationship. 

How often have the words relationship and side piece occurred in a corpus of books? (source: Google Ngram Viewer )