The ornamental design for a refillholder for electrictoothbrush, as shown and described.
FIG. 1 is a perspective view of a refill holder for electric toothbrush;
FIG. 2 is a front view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom view thereof.
Broken lines are used in the drawings to designate environmental structures or parts of a refill holder for electric toothbrush that form no part of the claim.