The ornamental design for a releaseably engagedarmchaircushion, as shown and described.
FIG. 1 is a perspective view of an armchair cushion;
FIG. 2 is a top plan view of an armchair cushion;
FIG. 3 is a bottom plan view of an armchair cushion;
FIG. 4 is a right side view of an armchair cushion;
FIG. 5 is a left side view of an armchair cushion;
FIG. 6 is a front side view of an armchair cushion; and,
FIG. 7 is a bottom side view of an armchair cushion.
The foregoing FIGS. 1-7 consist of photographs of the claimed design. These photographs are not intended to indicate any explicit colors associated with the claimed design. In addition, in some of the photographs certain items are depicted, such as shadows. Said items are not part of the claimed design and are therefore disclaimed. Furthermore, the broken lines form unclaimed boundaries of portions of the claimed design.