The ornamental design for a knifesheath, as shown and described.
FIG. 1 is a front and side perspective view of a knife sheath;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is an enlarged bottom plan view thereof; and,
FIG. 5 is an enlarged top plan view thereof.
The features shown in broken lines represent environmental subject matter that are for illustrative purposes only and form no part of the claimed design.