What is the meaning of Hip_hip_hooray?

An exclamation of congratulations or celebration, especially in response to a call for "three cheers for" the person. Often the person calling for the cheers will yell "Hip! Hip!", the crowd replying "Hooray!"

"Three Cheers, Hip-Hip-Hurrah, and Tom and Jerry, a Cheer-ful etymology" Peter Jensen Brown

WordOrigins.org Dave Wilton

Source: wiktionary.org