The ornamental design for an undergarment, as shown and described.
FIG. 1 is a perspective view of the undergarment of the present invention.
FIG. 2 is a front view of the undergarment shown in FIG. 1.
FIG. 3 is a left side view of the undergarment shown in FIG. 1.
FIG. 4 is a rear view of the undergarment shown in FIG. 1.
FIG. 5 is a right side view of the undergarment shown in FIG. 1.
FIG. 6 is a top plan view of the undergarment shown in FIG. 1; and,
FIG. 7 is a bottom plan view of the undergarment shown in FIG. 1.
The broken lines showing of an undergarment is included for the purpose of illustrating environmental structure and forms no part of the claimed design.