Is it ensure, insure, or assure?
1640s, transitive; 1680s, intransitive; from Latin convulsus, past participle of convellere (transitive only) "to pull away, to pull this way and that, wrench," hence "to weaken, overthrow, destroy" (see convulsion). Related: Convulsed (1630s); convulsing.
convulse con·vulse (kən-vŭls')
v. con·vulsed, con·vuls·ing, con·vuls·es
To affect with irregular and involuntary muscular contractions; throw into convulsions.