Sprue \Sprue\, n. [Etymol. uncertain.]

1. (Founding)
   (a) Strictly, the hole through which melted metal is poured into the gate, and thence into the mold.
   (b) The waste piece of metal cast in this hole; hence, dross.

2. (Med.) Same as Sprew.

Source: Webster's Revised Unabridged Dictionary (1913)