To repulse means to drive back or force away, like when an army repulses an enemy attack by fighting them off before they can break through the defenses. When a castle's defenders repulsed the invaders, they stopped the attack and sent them retreating. You might read in a history book that “the soldiers repulsed three separate attacks” or that “the invasion was repulsed.”

The word also means to cause a strong feeling of disgust or revulsion. Something that repulses you doesn't just seem unpleasant; it makes you want to move away from it immediately. A rotting smell might repulse you, or a particularly gruesome scene in a movie might be so repulsive that you have to look away. When something is repulsive, it triggers that instinctive reaction of “I need to get away from this now.”

These two meanings connect through the idea of pushing away: in one case, you're literally pushing back an enemy force, and in the other, something is so unpleasant that your mind and body want to push away from it. Something described as repulsive is deeply off-putting, whether it's a smell, sight, behavior, or idea that people find disgusting.