The ornamental design for a men's underwear, as shown and described.
FIG. 1 is a perspective view of the men's underwear showing the new design;
FIG. 2 is a front elevational view of the men's underwear thereof;
FIG. 3 is a rear elevational view of the men's underwear thereof;
FIG. 4 is a left side elevational view of the men's underwear thereof;
FIG. 5 is a right side elevational view of the men's underwear thereof;
FIG. 6 is a top plan view of the men's underwear thereof; and,
FIG. 7 is a bottom plan view of the men's underwear thereof.
The broken lines shown in the drawings depict portions of the men's underwear that forms no part of the claimed design. The human figure in FIG. 1 depicts environment and forms no part of the claimed design.