The ornamental design for toothbrush, as shown and described.
FIG. 1 is a rear elevation view of my new design for a toothbrush wherein the subject matter shown in dashed lines is disclaimed as forming no part of the claimed design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a left side elevation view thereof, the right side being a mirror image thereof;
FIG. 4 is a rear elevation view of an alternate embodiment, with the shading designating tonal contrast, wherein the subject matter shown in dashed lines is disclaimed as forming no part of the claimed design;
FIG. 5 is a front elevation view thereof; and,
FIG. 6 is a left side elevation view thereof, the right side being a mirror image thereof.