Green’s Dictionary of Slang

(UK society) aristocratic younger sons, impoverished through the inequalities of primogeniture, who are forced to live on their wits.

1887		Daily News 26 Sept. in Ware (1909) 49/1: When [the younger son] finds himself without the legitimate means to live and enjoy life, as he has been trained to do, he must either find illegitimate means or else join that party which has earned for itself the unenviable name of the broken brigade.
1897		London Eve. Standard 25 Sept. 3/2: New Books: [...] ‘One of the Broken Brigade’ by Clive Phillipps Wolley.