An ass-clap is the proper term for a the following action.

You see your best friend. You proceed to high-five your best friend. As your palms connect, continue on to slap your friend on the ass cheek. Proceed to bask in the awkward awesomeness.

Great for making a pass at your straight/gay teammates after a score.

Also the ultimate show of complete adjustment to your friend and the ultimate hand-shake/high-five no one dare copy.