niggling vs subnormal

niggling

adj

  • Irritating. 

  • Petty 

noun

  • A small irritating sensation. 

subnormal

adj

  • denormal 

  • Less than normal. 

noun

  • A person whose abilities are less than normal. 

  • That part of the axis of a curved line which is intercepted between the ordinate and the normal. 

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