er vs um

er

intj
  • Said when hesitating in speech. 

noun
  • The name of the Cyrillic script letter Р / р. 

verb
  • To utter the word "er" when hesitating in speech, found in the phrase um and er. 

um

intj
  • Expression of hesitation, uncertainty or space filler in conversation. See uh. 

  • Dated spelling of mmm. 

verb
  • To make the um sound to express uncertainty or hesitancy. 

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