castling vs quid pro quo

castling

noun
  • One that is cast. 

  • A miniature cast or mould. 

  • The act of constructing a defense structure in Japanese chess in which the king (玉) is positioned in a certain way so that it is protected by pawns (歩) and silver general(s) (銀) and/or gold general(s) (金) often with an additional knight (桂) and lance (香車). 

  • A move in which the king moves two squares towards a rook, and the rook moves to the other side of the king; the action of the verb to castle. 

quid pro quo

noun
  • An equal or fair transaction or series of process of exchange transactions; tit for tat. 

  • Something which is offered or asked for in exchange for something else. 

  • Something which is understood as something else; an equivocation. 

  • Substitution of one drug for another. 

  • A usually non-monetary exchange transaction, or series or process of exchange transactions. 

  • Sexual harassment in which a person in a workplace implicitly or explicitly requires sexual favours in exchange for something. 

How often have the words castling and quid pro quo occurred in a corpus of books? (source: Google Ngram Viewer )