The Lorentz Contraction is a relativistic phenomenon. As a moving object approaches the speed of light, its measured length contracts in the direction of motion. In the limit that it reaches the speed of light, it will have no length at all!

In the case of Fisk the fencer, the limerick is saying that his lunge is so fast that his epeé (a standard fencing weapon) moves forward at the speed of light. This causes the epeé to collapse into a disk with no thickness.