E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Broach.
To broach a new subject. To start one in conversation. The allusion is to beer tubs. If one is flat, another must be tapped. A broach is a peg or pin, and to broach a cask is to bore a hole in the top for the vent-peg.
