didder vs unnerve

didder

verb
  • To rattle or shiver. 

unnerve

verb
  • To make somebody nervous, upset, alarm, shake the resolve of. 

  • To deprive of nerve, force, or strength; to weaken; to enfeeble. 

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