an expression said when greeting an acquaintance or congratulating said acquaintance for a remarkable deed. hitting the rock is slowly replacing shaking hands as it is quicker, more hygienic & can be performed several times in a meeting (because of its brevity, it does not seem excessive when performed multiple times).
To properly hit the rock, one must say "...hit the rock." Then extend one's fist & gently touch the other party's clenched fist.
Yo, dude, hit the rock!
Hey, this rock isn't gonna hit itself!
Eeeyy, nice one, hit the rock!