To experience violent, involuntary muscle contractions, or to shake something vigorously with sudden movements.
From Latin convellere (con- = together + vellere = to pull or pluck), originally describing the violent pulling action of muscles during seizures, later broadened to any violent shaking.
The root vellere (to pull) is also in 'repel' and 'expel'—all describing intense pulling actions, so when you convulse, your muscles are literally 'pulling together' uncontrollably!
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