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1.
A wave where people in a crowd throw their hands in the air (and often rubbish) in a consecutive manner moving around a stadium. Often seen at cricket and rugby games. Sometimes when the game is really exciting (or boring).
Hey look Ford, there's a hell mexican wave happening.
by Pete April 03, 2004
 
2.
The act of nodding one's head at a friend or aquintance instead of the traditional wave.
Katy: Why does Steve never wave at me when I wave at him?
Maria: He did, look at his head, see he gave you a mexican wave.
by Joey Maton August 02, 2006