The ornamental design for a jock short, as shown and described.
FIG. 1 is a front exterior view of a jock short showing my new design;
FIG. 2 is a back exterior view;
FIG. 3 is a front interior view; and,
FIG. 4 is a back interior view.
Those portions of the figures that are presented in broken lines merely illustrate the environment of the design and are not part of the claimed design.
Broken lines shown in the drawings depict portions of the jock shorts that forms no part of the claimed design.