biker's nod
Greeting common among cyclists passing each other on the road. If you're biking along and can't give a proper "Hello and good day", you can just give someone a biker's nod. It also says "I acknowledge that I am one of your biking cohorts. We sure are a superior group of folk."
I couldn't lift my hands off the handlebars to say hi to that guy passing me on the road, so I gave him the biker's nod instead.