The ornamental design of an undergarment, as shown and described.
FIG. 1 is a front view of an undergarment;
FIG. 2 is a rear view of the undergarment of FIG. 1;
FIG. 3 is a side view of the undergarment of FIG. 1; and,
FIG. 4 is a top view of the undergarment of FIG. 1.
The broken lines depicting a user and front shape of the undergarment are for the purpose of depicting the environment and use of the undergarment and form no part of the claimed design. The broken lines in FIGS. 2 and 3 depict a back clasp and stitching of the undergarment, respectively, and form no part of the claimed design. The small broken lines in FIG. 1 depict stitching of the undergarment that forms no nart of the claimed design.