While having doggy-style sex (anal or traditional) the womans head is kept just above the rim of a toilet. When the man cums he dunks her head in the toilet and flushes resulting in a swirly.

The physical act of pushing a person's head into the toilet and pulling the chain (or pressing the button, for all you modern cistern types).

Suitable initiation/punishment for freshmen, especially new attendees of upper crust boarding schools (making the "Royal" especially apropos).

An act of oral sex performed on a male partner while seated on an open active comode during use. Also referred to as crowning. Or polishing the kings septor .

2 parts crown, 6 parts cream soda, 1 part maraschino cherry juice, 2 maraschino cherries. AKA The Velvet Crocodile. Fuckin excellence.

When You Jump In The Toilet, with or without the feces and iron in the toilet and then you fall backwards so far backwards that you pass the point of return and you flush the toilet.

when the queen or princess uses a douche

royal flush is the gambling game played by the bad guys in old bollywood movies, and the one where there is ample scope to cheat by having a couple of aces up your sleeve.
The winning hand in this game is also called the royal flush, which is getting three cards of the same type. Getting three aces is the strongest hand in the game.