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3.
A variation on the Dirty Sanchez sexual move, where the first participant eats a box of Lucky Charms 2-3 hours beforehand. This results in a more colorful and amusing feces.
Josh (on the phone): Hey girl, I was thinking we could try out the Mexican Rainbow
Girl: Sure!
Josh: Awesome, let me just go to Walmart and pick up some Lucky Charms. Ben's going to be so jealous when he finds out!
by Biros August 20, 2008
 
1.
Rainwater mixed with oil from several junky mexican owned vehicles leaving a parking lot, creating a rainbow-like effect on the pavement.
José and his homies left a huge Mexican Rainbow in Wal-Mart's parking lot.
by B_Rain March 10, 2011
 
2.
When a person eats really shitty Mexican food so that a few hours later he or she - but mostly he, since girls don't poop - simultaneously shits and vomits from both ends. Once the purging begins, the person will get on their hands and knees, and the spewing will look like a rainbow.
Man, last night I went to Vallartas Mexican Restaurant and I had the worst taco and refried beans EVER! When I got home I redecorated my bathroom with a Mexican Rainbow. It was so gross!
by theduderanch June 28, 2011