Percussion comes from the word Percuss. To percuss something is to tap it or hit it sharply, which is how you play most percussion instruments.

To strike smartly; to strike upon or against; as, to percuss the chest in medical examination., To strike or tap in an examination by percussion. See Percussion, 3.

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I think you mean pertussis. That is whooping cough. To percuss is to tap on a body part to listen to the sound. Or in music, percussion instruments are things like drums .

Your question is missing some information. The normal liver always crosses the midclavicular line. The missing information: how many centimeters did it percuss to at the midclavicular line.