Anvilling refers foremost to the illegal practice of ignoring a hit which strikes a sheathed weapon and would have struck the target had the weapon not been present. It can also refer to similar acts, such as laying a weapon across a shield and claiming that it "parries" a shield-breaking red weapon hit.
Anvilling is specifically prohibited by the Book of War:
3.7.3. Blocking a Weapon strike by laying a Weapon against a Target Area and/or Shield is illegal.
3.7.4. Sheathed or otherwise worn Weapons cannot block attacks.
Some fighters argue that when a defender blocks an incoming attack by placing his own weapon in front of blow so that his own weapon slaps his body instead of the incoming attack, that this is also anvilling, and thus not a legal block.