The ornamental design for an armrest, as shown and described.
FIG. 1 is a perspective right view of an armrest showing my new design in full lines with the broken line showing of environmental association matter being for illustrative purposes only and forming no part of the claimed design, the perspective left view being a mirror image of said perspective right view;
FIG. 2 is a top plan view thereof;
FIG. 3 is a right side elevational view thereof, the left side elevational view being a mirror image of said right side elevational view;
FIG. 4 is a bottom plan view thereof; and,
FIG. 5 is a front elevational view thereof, the rear elevational view being a mirror image of said front elevational view.