In an all-boys environment, usually if you don’t personally know someone, or if you’re like acquaintances, you greet them with a dap. A dap is like if you were to shake hands, but there’s a little more to it. You like grab the guy’s hand, and you hold it lightly and casually, and it’s more up and down and slidey. There’s multiple kinds, but this is the typical form, especially when you’re meeting someone new. It’s called dapping. An example is your friend introduces you to one of their friends, and so you dap them up as a greeting, and say what’s up. I don’t know where I learned that, but I’ve probably been doing it since summer going into freshman year, or that’s when it became super normal.
This is my little brother talking to me as we sit casually together.
My informant/brother goes to an all-boys school, and is a masculine boy with a lot of male friends. He would be considered “bro-ey” or a jock. He is a freshman in high school.
Almost every American boy I have ever known does this. It’s an informal greeting, and a very masculine one. They don’t hug, but they always do this. Girls joke about it.