Snappers are various drugs injected with about the tiniest gauge needles on the market that inject directly into the penis and give you a hardon. Lots of porn industry types use those.
another meaning:
Used in motocross and maybe other motocycle sports.
When the rider gets a rough ride from the bike, in a kinda out of control stylee. From the term "tank slapper" as in getting slapped up a bit by the fuel tank.