The "truly wondrous fantasmagorically splendiferously magickal word hooroo" (credited to someone's Uncle Wally) echoes two likely sources.
It's a bit of a joyous exclamation, as in "Hooroo! hooroo!
hooray!" (Darby O'Gill), and it's also something of an owl's hoot, as in "the proper hooroo, hooroo" (The Sword in the Stone).
Guys would say "Time to Hoo a Roo" on parting and this gradually became shortened to Hooroo.
Shelia: No worries, mate, see you for the shearing competition tomorrow. Hooroo.