It may have other meanings, but " a Lady From Bristol" or "Lady of Bristol" is cockney rhyming slang for "pistol".
Taken literally, Bristol is a large, elegant city in the southwest of England. Trouble is language/slang is forever changing, and in this country it changes from region to region. So while the phrase above might be used in London it would be rarely used in Scotland. The meaning is also often tied into the context of the sentence it was originally used in, making the phrase even harder to decipher - which was the reason cockney rhyming slang was used in the first place...