disparage vs slag off

disparage

verb
  • To ridicule, mock, discredit. 

  • To dishonor by a comparison with what is inferior; to lower in rank or estimation by actions or words; to speak slightingly of; to depreciate; to undervalue. 

slag off

verb
  • To talk insultingly to or about someone or something; usually one who is not present. 

  • To purify by melting and removing slag. 

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