The brown bomber and a dose of salts was a nickname for the combination of senna tablets and milk of magnesia.
The senna was a stimulant laxative, and the milk of mag was a ‘saline’ laxative (draws fluid into the GI tract from surrounding tissue).
The combination would set the average patient into spasms of lower intestinal cramping and diarrhea, dramatically resolving constipation.
The combination could actually be dangerous for some people with severe diarrhea, resulting in ruptured colons. It is no longer used.
A brown bomber is a senna based laxative. A dose of salts means a dose of epsom salts. In Cool Hand Luke” it was used to clean Luke out and make room for the 50 eggs.