So ...
Any proficient weapon counts as Monk weapon ...
That seems a bit weird, doesnt it?
Eh not really, if it's a weapon it's probably had a martial art made after it.
Who doesn't know the art of axe-fu...
There were some weapons that very much resembled axes at the place my kids used to study TKD. Really any farming implement has a long, long history of use a a weapon, and weapons studied in martial arts frequently lean more towards these than martial-use-only weapons