A skateboarding term, used to denote a type of grind wherein the back truck and sometimes tail of a skateboard come in contact with the surface being grinded.
To perform a 5-0 grind, first become comfortable with 50-50 grinds and manuals. Approach the object at a medium to fast speed, at about a 45-30 degree angle. Ollie, and land as you would with a 50-50, but apply a bit more pressure on the beginning of the tail to keep the front trucks up. A bit of pumping may be required if you find yourself moving too slow, as you would in a manual. It's okay to let the tail touch, and many professionals do it all the time. From here, most people 180 shove-it out, or kickflip or something else cool like that. However, turning off at the end of a curb or ledge seems to work just fine.
As far as I can gather, the 5-0 (pronounced five-oh) has its roots seated in surfing, as the whole of skateboarding does. Hawaii Five-0 may be the inspiration of the term, since skateboarders like to sit around and watch tv instead of working. =)