1. A variety of rattlesnake characterized by its unique sideways movements. I nearly jumped out of my skin when that sidewinder sped past in front of my horse.

2. slang By extension, an unscrupulous, despicable, or deceitful person. Primarily heard in US. That sidewinder tricked me into investing my life savings in his hedge fund, promising that he would make me a fortune with it. Now I'm left with nothing! These politicians are all sidewinders, exploiting people's fear and anger to further their own careers.