subtle vs unidentifiable

subtle

adj
  • Hard to grasp; not obvious or easily understood. 

  • Tenuous; rarefied; of low density or thin consistency. 

  • Barely noticeable, not obvious, indistinct. 

  • Cleverly contrived. 

  • Cunning, skillful. 

  • Insidious. 

unidentifiable

adj
  • Difficult, if not impossible, to identify or name 

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