hello vs what's up

hello

noun
  • "Hello!" or an equivalent greeting. 

intj
  • A greeting used when answering the telephone. 

  • A call for response if it is not clear if anyone is present or listening, or if a telephone conversation may have been disconnected. 

  • An expression of puzzlement or discovery. 

  • Used sarcastically to imply that the person addressed or referred to has done something the speaker or writer considers to be foolish. 

  • A greeting (salutation) said when meeting someone or acknowledging someone’s arrival or presence. 

verb
  • To greet with "hello". 

what's up

phrase
  • What's happening? What are you doing? 

  • What’s the matter? What is the problem? What is wrong? 

  • A casual greeting with a meaning similar to hi or nice to meet you. 

How often have the words hello and what's up occurred in a corpus of books? (source: Google Ngram Viewer )