An anklefuck is any sort of pitfall or perilous abyss in the ground where one may experience quite a bit of pain in their leg muscles upon falling into, hence being referred to as "fucking one's ankle" and possibly quite a bit more. Although it may have numerous uses and examples to be had elsewhere (i.e. a method of level design to designate where players can or cannot go in a given area, or something nature has formed in real life,) it is oft used in the context of Minecraft as a method of griefing, whereupon one person cleverly disguises a pit in the ground for others to fall into and otherwise become injured by or die in.
The verb form simply refers to the action of being similarly injured or killed by a fall.
Watch out for that Grand Canyon, Bob. It is quite the anklefuck.
Watch out for that Grand Canyon, Bob. You'll get anklefucked six ways from Sunday if you fall in.