When you do something involuntarily, you do it despite not wanting to. You might involuntarily start to giggle during your cousin's Star Wars-themed wedding ceremony.
Use the adverb involuntarily to describe things that are done against your will or without your control. You'll involuntarily take a swim if your brother pushes you in the deep end of a pool; when you have a twitch in your eye, you blink involuntarily. In either case, the word involuntarily combines the Latin prefix in, "not or opposite of" with voluntarius, "of one's free will."