The ornamental design for a posturesupportundergarment, as shown and described.
FIG. 1 is a front view of a posture support undergarment according to a first embodiment of the design;
FIG. 2 is a rear view thereof;
FIG. 3 is a right side view thereof. The left side view is a mirror image of the right side;
FIG. 4 is a front perspective view thereof;
FIG. 5 is a rear perspective view thereof;
FIG. 6 is a front view thereof without the environmental structure (of the human body);
FIG. 7 is a flat view of the design;
FIG. 8 is a front view of a posture support undergarment according to a second embodiment of the design, without sleeves;
FIG. 9 is a rear view thereof;
FIG. 10 is a front view thereof without the environmental structure (of the human body); FIG. 11 is a flat view of the design; and,
FIG. 12 is a front view of a posture support undergarment shown in FIG. 1 in an alternate configuration of use;
Broken lines are for the purpose of illustrating portions of the environmental structure or alternate environment of use and form no part of the claimed design.