Honorless Target is a buff, which was introduced in Patch 1.4, but not made visible until patch 1.41.
Blizzard says (in the patch notes for patch 1.41):
Honorless Target - Players who just land from a gryphon/wyvern, or step out of an instance will gain this buff for 30 seconds during which this target will give no contribution points if killed. This was originally implemented in the previous patch, but was kept invisible.
It has been noticed that this buff also occurs when players have just used their Hearthstone. Additionally, this status occurs if a player's connection times out, yet does not result in the character's death.