When a skateboard begins to tightly swerve back and forth from left to right with increasing velocity until the skateboard begins to undulate so rapidly that the rider is usually thrown from the board.
I was doing some downhill skateboarding and I went so fast that I got the speed wobbles and got pitched into the street.
A state of shakiness a rider experiences while hitting a high speed on a board typically accompanied by an adrenaline rush and/or a state of panic.
Dude: Dude, bro, I would have totally stuck that turn but I got the speed wobbles half way down the hill and I knew it was time to bail!
Dude, bro: Poser...