I just remembered one more meaning for "thimble". When I was a boy, my father fixed up a gym in what had been my grandfather's hay loft. We had a pool table, basketball court (naturally not regulation size) and he put up a pair of trapezes. To make these safe, in addition to splicing loops in the rope ends connected to the roof rafters, the loops were constructed around tear-shaped half-round channels called thimbles that went over the hooks in the rafters, to strenthen the loops of rope. The same thimbles are used on sailing ships where there is a loop of rope subject to heavy loads over long periods.

A difficult thing to describe, but I tried hard to make it clear.