1 hour ago, aditus said:

This term always gave me pause when I encountered it. It proves language is a living, breathing thing. For me bromide is and always will be : a chemical compound containing a bromide ion or ligand.  (Br−) *shrugs*

6 minutes ago, Valkyrie said:

That was my first thought too. 

It's exactly where it comes from.

Until the mid '70's potassium bromide was used as a painkiller and sedative for almost everything. Hence a bromide will literally put you to sleep.