v. To force or drive (a fluid) into something: inject fuel into an engine cylinder; inject air into a liquid mixture. v. Medicine To introduce (a drug or vaccine, for example) into a body part, especially by means of a syringe.v. To treat by means of injection: injected the patient with digitalis. v. To introduce into conversation or consideration: tried to inject a note of humor into the negotiations. v. To place into an orbit, trajectory, or stream.