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 thereof;
FIG. 3 is a front interior view thereof; and,
FIG. 4 is a back interior view thereof.
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.