The ornamental design for a thongsock, as shown and described.
FIG. 1 is a perspective view of a thong sock for a right foot showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear 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 bottom plan view thereof;
FIG. 7 is a top plan view thereof;
FIG. 8 is a bottom left perspective view thereof;
FIG. 9 is another bottom left perspective view thereof;
FIG. 10 is a bottom right perspective view thereof; and,
FIG. 11 is a top plan view showing a paired thong sock for a left foot.
The broken line showing of the paired thong sock is included for the purpose of illustrating environmental structure and forms no part of the claimed design.