Putting a one handed weapon in the monk's offhand slot will increase his accuracy with the fist attack.
How to reproduce:
Create a new monk. His starting accuracy with fists will be a certain value (in my case 30\30).
Go to Heodan and buy a dagger. Equip the dagger in the offhand slot. His accuracy will now be 42/47
See also: https://steamcommuni...57398960796140/ (thread not made by me)