Something that is totally Awesome.
This term would be used among bros when something exciting happens.
Statement: Breh, I got some gnarly bud.
Reply: headmastic dude!
What something is when its totally headmastic.
It can be used in place of awesome, richter, crazy, rediculous, delicious, bangerang,the best, impressive, brodaceous...
Usually this term is used by bros to describe something awesome.
Whats up bro, You'll never believe it, i hooked up with stacy last night!