The ornamental design for a rearbumper for an automobile, as shown and described.
FIG. 1 is a front perspective view of a rear bumper for an automobile showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a bottom view thereof; and,
FIG. 4 is a right-side view thereof, with the left-side view being a mirror image.
The rear surface has not been shown and is not intended to be part of the claimed design. Any small portions of the rear surface that may be seen in the front perspective view are not part of the claimed design.