What does "滿腦門子的黑線" mean? 腦門子


would be forehead.


  黑線


is black lines.

Does it mean something like "veins popping on his forehead"?
It's not popping veins, but cold sweat for frustration or helplessness, usually as a response to something completely out of the line or out of the context.

Wikipedia:


  Parallel vertical lines with dark shading over the head or under the eye may represent mortification, fatigue, or horror. If the lines are wavy, it may represent disgust. A far cuter way to represent frustration/mortification is (mainly for female/ young female characters) they tend to puff out their cheeks while their line is delivered in a gruff voice, an elongated 3 showing puffed lips to show that puffed look more.




Popping vein is visualized and then character-ized as '头上一个大大的井字'.