Wikipedia
A buttonholer is an attachment for a sewing machine which automates the side-to-side and forwards-and-backwards motions involved in sewing a buttonhole.
Most modern sewing machines have this function built in, but many older machines do not, and straight stitch machines cannot sew a zigzag stitch with which buttonholes are constructed. A buttonholer attachment can create buttonholes from any sewing machine capable of making a lock stitch. (That is not to say, however, that some industrial buttonhole machines cannot employ a chain stitch, especially to create the purl when making keyhole buttonholes).
The buttonholer's adaptor attaches to the machine's presser bar, replacing the presser foot. In its place the buttonholer employs a cloth clamp with teeth on the underside to hold the material firmly whilst manipulating the cloth side to side and forwards and backwards. Driven by the up and down stitching motion of the needle bar via the fork arm straddling the needle clamp, it executes the series of movements to complete a buttonhole by moving the material rather than by moving the needle position. Buttonholers usually include a metal feed cover plate to cover the machine's feed dogs, so that they do not interfere with that of the buttonholer, though some machines allow the feed dogs to be disengaged or ‘dropped’.
Usage examples of "buttonholer".
There must have been something in his tone which warned the Buttonholer, for the man let loose his grip and backed away.