A few other meanings missing from Webster:

  1. n. The basic ingredients of a recipe (e.g. milk, flour, etc) that uses all basic ingredients, as opposed to prepared dishes or mixes purchased from a store. It is especially used for baked goods, e.g., My mother made this pumpkin pie from scratch.
  2. (Slang)n. Money; as if the stuff simply lies around waiting for someone to "scratch it up" like a chicken would (cf. transitive verb definition 4 below).
  3. "Old Scratch", a quaint colloquial name for the Devil.
  4. v.t. To withdraw an entry from a (horse) race. (cf. the obsolete "to scratch a ticket" below)
  5. v.i. (Pocket billiards, aka "pool") A shot that results in the cue ball going into a pocket, or off the table. A player who scratches loses his or her turn; in eight ball, a player who scratch while trying to sink the eight ball immediately loses the game. In very strict games "scratch" can mean fumbling the shot so that the cue glances off the eight ball, or even the draconian definition of Webster 1913 below (a shot not called beforehand).