Term used in the cycling community for a really gung-ho athletic rider, the kind with an intense training regimen and a "need for speed." Named for their habit of dropping the hammer
, or launching into unnecessary bursts of speed, particularly blowing off weaker riders in recreational group rides. In extreme cases, they've been known to blow off traffic lights and laws just to go fast. Too often matching the stock image of the athletic, lycra-clad scofflaw, they're often blamed for perpetuating a lot of ugly stereotypes about people on bikes.
"I get to the intersection on my bike, and just when I get the light, some hammerhead blows off the intersection right across my bath! He cuts me off, AND all those cars. Then they all start honking at me, even when I signal properly, because they think I'm a jerk, too. I'm just trying to get to work!"