It means “having the quality or nature of an antidote to poison.” The word was introduced into English in the seventeenth century in the now obsolete form alexipharmac, a noun meaning “an antidote against poison”. This comes directly from a Greek word which is a compound of alexein, “to ward off”, and pharmakon, “poison”. This word alexipharmac was so often used attributively that users came to think it was an adjective, and so decided it ought to be spelled with the –ic ending instead. The Greek prefix alexi– is rare in English, turning up only in words such as alexipyretic, “helping against fever”; alexiteric, “having the power to ward off contagion”; and alexin, an obsolete word for what is now called a complement, a system of proteins in the blood, particularly one which guards against infection. Pharmakon could mean a drug as well as a poison (a disturbing equation of ideas to the modern mind), and is much more familiar as the origin of our words in pharmaco– such as pharmacology.