Like a thin washer with serrated inner diameter that you push over a
shaft
to retain something on it. From memory they are dished slightly on one
face (the side that goes on first) so that the serrated bits bite into
the
shaft material.

Starlock ?

Yes that's the monkey

Part of the large family known collectively as ping****its,
of small things capable of converting their stored potential
energy into kinetic energy in the blink of an eye, or sometimes
if you're unlucky, without the blink of the eye...

The domed one sided starlock jobbies i've always had to remove come off
quietly with not much fuss.... but the little *******s are bending their
insides during that process, so they never want to go back on without a
fight,

There's probably a proper tool to remove them in a re-useable condition, and
a couple of screwdrivers wiggled about between them and the non rotating
parts of the item isnt it.