Dates of Origin
New words, idioms, expressions etc. are constantly being added to language. This is known as a first citation or attestation, and it must appear in credible correspondence, print or broadcast media. Only when it has appeared in such media, can a new word or idiom become part of language.
First citations or attestations are sometimes difficult to determine because new evidence of first occurrences continue to emerge over time. Therefore, except in those instances where a first citation or coinage is well known and well documented, only the approximate dates of origins can be given.
Writers of historical novels in particular should be aware of when idioms first enter the language. For example, Patrick O'Brian, author of the Aubrey/Maturin novels, set in the Napoleonic Wars, has a character using the idiom 'Bob's your uncle'* in 1804, some 100 years before its first known usage.
*Patrick O'Brian Post Captain, Harper Collins, paperback edition (2002) p. 291.