A quick lesson in Helhiem culture:

Bull Headed: For the beastmen of Helhiem, the term 'Bull Headed' is a highly negative term to use and is at times an insult worth killing someone over. Only one tribe on the planet contains bullmen and that is the Minotaur tribe that worships the Dark God Khorne; While they are insanely powerful melee fighters, the Minotaur of Helhiem are driven completely insane by their blood lust to the point where they will randomly murder, skin and consume allies and siblings without warning in an attempt to temporally sate their raging blood lust (If the victim of this betrayal is lucky, it will happen in that order). 

To call something Bull Headed is to say that it is somehow akin to one of these bloodthirsty, psychotic monsters. While it can be used as a serious insult, it can also be used as an extreme way to express warning or concern; A shock tactic of last resort to express just how dire the situation is. For example, to call a plan Bull Headed is to effectively say that it is completely psychotic and is likely to result in everyone dying horribly or worse.