When you have sex high in the mountains especially in the open on the ground, such as in the mud. It has also been used to reference anal sex in the mountains as well. It was originally used in the rocky mountains but has been adapted as a more loose term for the act.
The act of 12 laxatives and shitting on a water slide and sliding down it with a dildo on your head to then force your whole head into a woman’s vagina at the bottom
Me a d your mum did a “Mexican mud slide”last night