The ornamental design for a cutterknife, as shown and described.
1. Cutter knife
1.1 : Front perspective view
1.2 : Back perspective view
1.3 : Front
1.4 : Back
1.5 : Left
1.6 : Right
1.7 : Top
1.8 : Bottom
In views 1.1-1.8, the lighter grey, blurred portions enclosed by the broken lines are for illustrative purposes only, and form no part of the claimed design.