Origin: Resessive Mexicans.
A very kee and oss Mexican with white skin and red hair which carries many strange resessive traits. Mexes often do yard work or have pressure washing businesses. They generally like to squeeze into trucks with 3 or 4 other Mexicans in them. It can be used as a name or a noun. When generically used, it can also simply mean Mexican, but it generally refers to sweaty Mexicans doing yard work.
"Hey, look there's 18 Mexes in that Toyota truck! How oss!"
"Sam's a mex."
"Hey, Mex, are you done mowing my yard?