Short for barbiturates.
Barbiturates are drugs that act as central nervous system depressants, and, by virtue of this, they produce a wide spectrum of effects, from mild sedation to total anesthesia. They are also effective as anxiolytics, hypnotics and as anticonvulsants. They have addiction potential, both physical and psychological. Barbiturates have now largely been replaced by benzodiazepines in routine medical practice mainly because benzodiazepines are significantly less dangerous in overdose.
Barbs will mess you up. They are the third most dangerous drugs (in terms of dependency and physical harm) next to heroin and cocaine.