The majestic manifestation of the RMS-Rhone-sized erection (typically the penile variety) after discovering something larger than life, incredible, fantastic, magnificent, or impressive.
A shorter means to describe something as larger than life, incredible, fantastic, magnificent, or impressive.
Holy Shit! Check out that double rainbow!!! It's the Rhone boner of rainbows, man! You've gotta see this!
Wow! Scuba diving in between the 10 ft propeller of that boat wreck totally gave me a Rhone boner.