The ornamental design for a weightbag, as shown and described.
FIG. 1 is a perspective view of a weight bag showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a left view thereof; and,
FIG. 7 is a right view thereof.
The broken lines showing of a logo in FIG. 1, FIG. 4, and FIG. 6, and the broken lines showing of stitching in FIG. 1, FIG. 4, and FIGS. 6-7 are included for the purpose of showing environmental structure and forms no part of the claimed design.