The ornamental design for a rearbumper for an automobile, as shown and described.
FIG. 1 is a top, front and right side perspective view of a rear bumper for an automobile showing my new design;
FIG. 2 is a bottom, front and left side perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a right side elevational view thereof; and
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken line showing of the rear bumper for an automobile is for purpose of illustrating environmental structure and forms no part of the claimed design.