My father tells me that "whoopie" was used by the old timers for automobiles, & before that, a buggy drawn by horse(s). Daddy says that one had to "whoop the horses to get 'em goin'." [I my day, a "whoopin'" was a hard spanking.] His Daddy & his "Granpap" came from around the border of Texas & Oklahoma, and I have heard a great deal of oldtimey talkin'.   I heard his father as well as my Irish Grandad use the term "whoopie" meaning a vehicle. My Irish Grandad never drove any whoopie except for one drawn by a horse or mule.
   Notice the term "Granpap" short for "Grandpappy?" The old timers--especially the Irish but others too, according to my Daddy, used the terms "Pappy" and "Mammy" for their parents, thus Grandpappy & Grandmammy.   As in "Mammy Yokem" & "Pappy Yokem" of the Lil Abner comic strip.