blue pill
/ˌbluː ˈpɪl/Now historical
noun
1 Pharmacology. Any of various medicinal preparations containing metallic mercury or a compound of mercury, used especially in the treatment of syphilis and disorders of the liver, and as a purgative; (as a count noun) a tablet of such a preparation.
2 US slang. A bullet. Now historical.
Origin
Late 17th century; earliest use found in Gideon Harvey (c1636–?1702), physician. From blue + pill.