Clearly, this skink has lost its tail one day. 
Many lizards have developed a defense mechanism called "autotomy," or, more simply, they are able to drop their tail when they are threatened by a predator.
Tail loss, though natural, does cause some issues for the lizard.
First, it will generally affect the lizard's sense of balance, and it may have trouble climbing or walking regularly until it adjusts to the lack of a tail.
For a lizard that stores fat in its tail, it also results in the loss of critical fat deposits. 
Many lizards will regenerate their tail, especially if they are young lizards.
However, the tail is generally not the same length or color as it was before, and it may have abnormal scales or patterns.

Reserva Ecologica Taricaya, Puerto Maldonado, Peru, 24 June 2009