not sure of it's accuracy, but just came across this info as to the origin of "off his rocker" while researching printing methods.....
Mezzotints are produced on copper plates. The entire surface of the plate is roughed
with a tool, shaped like a wide chisel with a curved and serrated edge - the Mezzotint
Rocker. By rocking the toothed edge backwards and forwards over the plate, a rough
burr is cast up which holds the ink. Once this is completed, a drawing can be
transferred onto the plate, using carbon paper. When printed, the textured ground
reads as a uniform dark; the areas to be lightened are scraped and burnished -
therefore, working from dark to light - a reverse technique to etching and engraving.
Little can compare the Mezzotint in the richness of its blacks; it is unique among the
intaglio printmaking processes.
The preparation of the plate can take 15 hours or more before the artist can start work
on the design, but the beautiful, soft velvety finish is so unique to the mezzotint
process that it more than justifies the skill and patience involved. In the 18th Century,
small boys were employed to 'rock' the plates up and the extreme tediousness of the
work, combined with the poor pay and working conditions, sent many of the poor
things into mental decline, hence the term "off one's rocker".