Definitions:
a device consisting of a bar or wheel with a set of angled teeth in which a cog or tooth engages, allowing motion in one direction only

a situation or process that is perceived to be changing in a series of irreversible steps

operate by means of a ratchet

cause something to rise or fall as a step in a steady and irreversible process

(especially in african american usage) unattractively coarse, disreputable, or unfashionable (typically used of a woman)