side piece vs wifelet

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. 

wifelet

noun
  • A long-term girlfriend; a mistress (the “other woman” in an extramarital relationship). 

  • A wife who is of small stature. 

  • A wife. 

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