@FAIL 

@fail <object> [=<message>]. <object> can be a thing, player, exit, or 
room, specified as <name> or #<number> or 'me' or 'here'. Sets the 
fail message for <object>. The message is displayed when a player 
fails to use <object>. Without a message argument, it clears the 
message. 

See also @OFAIL.