The ornamental design for an “armrest,” as shown and described.
FIG. 1 is a perspective view of armrest showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a back elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a reference view (a perspective view showing a chair with an armrest) thereof.
The broken line showing the remainder of the chair in FIG. 8 is for the purpose of illustrating environmental structure and forms no part of the claimed design.