above-board vs authenticate

above-board

adj
  • In open sight; without trick, concealment, or deception. 

adv
  • Honestly; openly. 

authenticate

adj
  • Synonym of authentic. 

verb
  • To render authentic; to give authority to, by the proof, attestation, or formalities required by law, or sufficient to entitle to credit. 

  • To prove authentic; to determine as real and true. 

How often have the words above-board and authenticate occurred in a corpus of books? (source: Google Ngram Viewer )