The term "bust" is one of the most negative labels that can be slapped on an NFL player. Usually given to early-round draft picks, it means not only did a player fail to meet expectations, but he massively underperformed in his career.
We're talking about guys who quickly flamed out of the NFL, like JaMarcus Russell, Johnny Manziel and Trent Richardson.
While it generally takes a couple of years to accurately identify such players as busts, the warning signs are often there immediately. For example, Richardson—who actually had a strong statistical rookie season—was good in space but regularly failed to find and hit open running lanes.