Soft Tommy, or white Tommy; bread is so called by sailors, to distinguish it from biscuit. Brown Tommy: ammunition bread for soldiers; or brown bread given to convicts at the hulks.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Tom Thumb * To-morrow Come NeverNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes