"Tranc" is a slang or truncated term used to refer to tranquilizers.
Used primarily in wild animal control and monitoring, it referred to the dose of tranqilizer carried in the projectile fired from a "tranc" rifle or pistol (firearm).
The term is used more commonly today to refer to any substance whose primary function is to tranquilize the subject, either through inducing sleep or impairing brain function to reduce fight or flight response to non-threatening levels