Here's the breakdown:
* Fettling is a process that refers to cleaning and finishing rough castings. This includes removing excess metal, sand, and other imperfections from the surface of the casting.
* Knife refers to the specific tool used in this process, which is a specialized blade with a curved or angled shape.
Therefore, a "fettling knife" is a knife specifically designed for fettling, the process of finishing metal castings.
The term's origin likely reflects its function, with "fettling" describing the task and "knife" identifying the tool.