jack nut

Meanings

1. A type of nut with a sheet metal body that mushrooms outward, on the far side of the hole through the substrate, when the screw is screwed in, thus providing anchoring with only one-sided access to the hole. It is nearly equivalent to the female portion of a molly bolt.

Etymology

From the idea that the nut is jacked into its deformed (expanded) state by a jackscrew-like action of the screw. In practice this step is often performed with a tool that screws in and backs out again, and then the final mating screw follows afterward.