flapjack vs slapjack

flapjack

noun
  • A pancake. 

  • A bar made of (though not limited to) rolled oats, butter, golden syrup, and brown sugar, baked in a tray. 

slapjack

noun
  • A type of pancake, or flapjack. 

  • A simple card game similar to snap. 

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