A scab is the hard, crusty layer that forms over a cut or scrape as it heals. When you skin your knee on the playground, your body immediately goes to work: blood cells rush to the wound, clot together, and dry into a protective covering. This crusty shield, the scab, keeps dirt and germs out while new skin grows underneath.

Scabs might itch as they heal, but picking at them is a bad idea. Underneath, your body is busy creating fresh skin cells. If you pull off a scab too early, you interrupt this process and might end up with a scar. The scab falls off naturally when the new skin is ready.

The word also has a completely different meaning in labor disputes. A scab is someone who crosses a picket line to work during a strike, when other workers have stopped working to demand better conditions or pay. This usage has been around for a long time and remains controversial and emotionally charged.