Hanging Offence Meaning

(idiomatic) A crime so serious that it is punishable by means of death by hanging.

Example: 1908, John Fox Jr., The Trail of the Lonesome Pine, ch. 26:
  Word reached the ear of the prosecuting attorney of the only testimony that could establish a motive and make the crime a hanging offence.
1999 June 18, Rod Beaton, "Astros winning despite obstacles," USA Today:
  Since when is it a hanging offense to criticize someone who's not doing the job he's paid generously to do?