The ornamental design for a lifthook, as shown and described.
FIG. 1 is a front elevation view thereof.
FIG. 2 is a rear elevation view thereof.
FIG. 3 is a left side view thereof.
FIG. 4 is a right side view thereof.
FIG. 5 is a top plan view thereof.
FIG. 6 is a bottom plan view thereof, and,
FIG. 7 is a perspective view of a lift hook showing my new design; and,
FIG. 8 is a reduced scale, environmental view with the broken line showing of chains being for illustrative purposes only and forming no part of the claimed design.