coulter
noun
coul·ter ˈkōl-tər 
: a cutting tool (such as a knife or sharp disc) that is attached to the beam of a plow, makes a vertical cut in the surface, and permits clean separation and effective covering of the soil and materials being turned under
Word History
Etymology
Middle English colter, from Old English culter & Old French coltre, both from Latin culter plowshare; akin to Greek skallein to hoe — more at shell
First Known Use
before the 12th century, in the meaning defined above