The ornamental design for the side portion of a stirrup of a pivotingankle brace, as shown and described.
FIG. 1 is an outer side view of the side portion of a stirrup of a pivoting ankle brace showing our design; the inner side thereof forms no part of the claimed design;
FIG. 2 is a rear view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a top view thereof; and,
FIG. 5 is a bottom view thereof.
The broken lines show portions of the ankle brace that form no part of the claimed design.