Thirteen: 13

The number 13 has a number of interpretations: 

Being the 13th letter of the alphabet, when worn on a bikers cut off, it is said to stand for marijuana or motorcycle. The assumption being, someone wearing a 13 patch is either a user or dealer of marijuana or other drugs, rather than 13/M simply standing for Motorcycle. Another definition of a 13 patch is that the wearer is his own judge & jury - this is due to 13 standing for 12 jurors and the additional number being the judge.
Though considered by many as being unlucky - The truth is, 
you make your own luck!