1580s, "a violent and involuntary contraction of the muscular parts of the body," from Latin convulsionem (nominative convulsio) "cramp, convulsion," noun of action from past-participle stem of convellere "to tear loose," from assimilated form of com "with, together" (see con-) + vellere "to pluck, pull violently" (see svelte).
Meaning "any violent or irregular (social, political, etc.) motion, turmoil" is from 1640s. Of laughter, 1735. Related: Convulsions; convulsional.