(n.) the first cut made with a saw
The first cut made with the blade of a saw, which often serves to guide the blade for the remainder of the cutting, is called the kerf.
That being said, the word kerf has a number of meanings in English, and if you’re a devotee of woodwork, handiwork, or even tree surgery then there’s a good chance you’ll have heard it used differently.
So as way as meaning the first cut or incision of a saw (spoiler alert: there’s a reason why we picked that definition for this tweet), a kerf can also be the lump of timber that is cut away; the bare, cut end of the branch of a tree after pruning; the flat, rectangular end of a piece of wood; the quantity removed by a cutting or a sawing; and the actual act of cutting itself. In the late Middle English period, it was also a fairly ludicrous name given to a group of pantrymen, or butler’s assistants:
A Kerff of Panteris.
The Book of St. Albans (1486)
The original initial-C spelling survived for a time—as too did a spelling more closely allied to the word’s roots, carf—but the initial K emerged in the Middle English period, likely due to Norse influence on the dialects of northern and eastern England. This spelling first emerged in the late 1300s, promptly ousted all others from the language, and has remained in use ever since.