OED

All over the world, distinguished consultants, advisers and contributors are working on amendments and additions to the Oxford English Dictionary,
"the accepted authority on the evolution of the English language over the last millennium…. an unsurpassed guide to the meaning, history, and pronunciation of over half a million words, both present and past…. traces the usage of words through 2.5 million quotations from a wide range of international English language sources, from classic literature and specialist periodicals to film scripts and cookery books”.

And anyone can join in. In the OED newsletter there are regular appeals for documentary evidence which is lacking, in particular the dates of the earliest authenticated use of words or phrases. They’re working on the letter P at the moment, and among the items listed in June are:
to piss on from a great height (v.: to humiliate utterly) 1992
poo(h) (n.: faeces, as a count noun) 1981
poo(h) (v.: to defecate) 1975
So if you have proof of the use of any of these earlier than the dates they give, let them know (at oed3@oup.com) and you could be contributing to the greatest work of scholarship in English since Samuel Johnson’s dictionary was published.

I have chosen the above three from this month’s appeals because there’s nothing like a bit of scatological research to help the day wind down, but of course such expressions are not the norm.
Among the appeals for June is also:
pork scratchings (n.) 1982
I would have thought that these existed in the nineteenth century, but surprisingly it seems the OED researchers haven’t yet found a reference to them before 1982. Can anyone help?
It’s no use just telling them that your grandfather used to talk about them all the time: there has to be a quotable document in which they are mentioned. Oh, and the editors already know about anything you can find with Google.

The BBC in conjunction with the OED is having its own word hunt on the same principle in preparation for a series on BBC2 next year. They’re looking for the earliest uses of – for example – bonk. The OED already lists six quotations for this (in the sense that you’re thinking), but has nothing before 1975. One of the more recent sources cited is the Daily Telegraph for 29th October 1986 with the quotation:
“Fiona..has become so frustrated that she has been bonking the chairman of the neighbouring constituency's Conservative association.”
Here is the full OED entry for bonk, which runs to 500 words. But you can't go on from there to the other half-million entries in the OED unless you have a subscription to the full OED Online, which will cost you £195 a year plus VAT (or $295 over there; the OED has a New York-based Editorial Unit to keep an eye on North American English, and are currently working on updates to the entries for pardner, Parker House roll, pat-down, pasta-fazool, and patootie).