The third-person singular present tense of convulse, meaning to cause violent muscle contractions or shaking in someone or something.
From the verb convulse plus the third-person singular present suffix -es, following standard English verb conjugation rules.
Simple word forms like 'convulses' are grammatical building blocks—they're the same action every time, just in different 'person' or 'time'—like saying 'I convulse, he convulses, we will convulse!'
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