hard to understand vs opaque

hard to understand

opaque

verb
  • To make, render (more) opaque. 

adj
  • Neither reflecting nor emitting light. 

  • Allowing little light to pass through, not translucent or transparent. 

  • Obtuse, stupid. 

  • Describes a type for which higher-level callers have no knowledge of data values or their representations; all operations are carried out by the type's defined abstract operators. 

  • Unclear, unintelligible, hard to get or explain the meaning of. 

noun
  • Something which is opaque rather than translucent. 

How often have the words hard to understand and opaque occurred in a corpus of books? (source: Google Ngram Viewer )