The ornamental design for an undergarment, as shown and described.
FIG. 1 is a front perspective view of an undergarment showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof; and,
FIG. 5 is a left side view thereof.
The broken line showing of portions of the body of a wearer is for illustrative purposes only and forms no part of the claimed design. The loops on the stripe that encircles the waist region represent a terry cloth appearance.