It is used rarely. I haven't heard this is so long. But you would use it to mock something that somebody believes is a big deal. For example: "I just got a participation trophee!" "Whoop-dee doo.."

In the past, it was a general expression of enthusiasm, like "Yow!" or "Yee haw!" It could be sincere. It was probably regional; when I hear it I think of the U.S. West and cowboys and rodeos. In the 1950s, it was the subject of a hit popular song, "Hoop-Dee-Doo" (variant spelling of the same phrase). Hoop-Dee-Doo, Hoop-Dee-Doo, I hear a polka and my troubles are through; Hoop-Dee-Dee, Hoop-Dee-Dee, This kind of music is like heaven to me! Hoop-Dee-Doo, Hoop-Dee-Doo, Has got me higher than a kite-- Hand me down my soup-and-fish, I am gonna get my wish Hoop-Dee-Doin' it tonight! One version: https://www.youtube.com/watch?v=YXtHyydM5Ac In the song, it's sincere. It means "I love to polka and I am really excited to be going to a polka dance." The expression... as well as polka music and the accordion... in U.S. culture have overtones of being old-fashioned, rusticated, unsophisticated. So nowadays it is more often heard in an ironic context. It's spoken with a mocking intonation and means "I am not enthusiastic about this at all." "I just heard that Dunkin' Donuts has started offering pumpkin spice coffee again!" "Well, whoop-tee-doo." Or "I could care less." Or "Big whoop." In each case, meaning "I don't think this is exciting."

It's not heard much these days.

It is slang used for sarcasm. If you use it, you will sound like a smart elick (obnoxious person). It will not win you many friends. :) It means something like, "well whatever you think" as if you don't care what the person thinks. "Well, whoop te do, tell me something that I don't know"