Mulish
/ˈmjuːlɪʃ/
Definition 1 of 1
Pro Tip 1/3Negative Connotation
Describes a negative trait, often suggesting unreasonable or annoying persistence. 
Despite the clear evidence, his mulish denial of the facts was frustrating.
Pro Tip 2/3Not Insulting
Can imply firmness without being offensive, but be careful with the context. 
His mulish dedication to his work is why he's successful.
Pro Tip 3/3Body Language
Often accompanies actions or facial expressions showing determination or defiance. 
Her mulish stance made it clear she wouldn't budge.