billman vs halberd

billman

noun
  • A man who uses, or is armed with, a bill or hooked axe. 

halberd

noun
  • A two-handed pole weapon, consisting of a long pole with an axe-like blade mounted on it (at a right angle like an axe, not on the tip like a spear), and (opposite the blade) typically a spike or hook. 

How often have the words billman and halberd occurred in a corpus of books? (source: Google Ngram Viewer )