Pronounced gib, hard g like in give.
What is it? It is a wedge shaped thing that is used to adjust the clearance in a dovetail slideway on a machine.
You have probaly seen lathe cross-slides: There is a dovetail way across the saddle and a matching dovetail slot in the cross-slide. The sides on the saddle way are parallel but the sides on the cross-slide way are not. One side is parallel to the direction of motion but the other side tapers slightly.
The taper is the same as that on the gib which is inserted in the gap. The sliding clearance depends on how far it is pushed in. Some gibs are adjusted with a bolt at each end, one pushing it in the other out. Actually it is the head of the bolt that bears against the gib. Other gibs have a single screw with the head in slot and the larger end of the gib so it can push and pull.
An open mind is a virtue...so long as all the common sense has not leaked out.