Flamboyant, Brilliant, Remarkable
い adjective
はなばなしい
その左利きの投手は、大リーグで華々しいデビューを飾りました。
That left‐handed pitcher made a brilliant major league debut.
華やか is "showy." Now we're doubling it, making it doubly showy. If you make something showy enough, it becomes gorgeous, but it also becomes pretty flamboyant. So, so, so flamboyant.
The reading comes from 華やか. Note the second はな becomes ばな, to be はなばなしい.