Cream Crackered

http://www.worldwidewords.org/qa/qa-cre2.htm
"I'm cream-crackered."

A cream cracker is a savoury dry biscuit, often eaten with cheese. Sometime in the past thirty years or so the phrase has become rhyming slang in Britain for knackered. That’s a slightly older slang term—there are examples going back into the 1950s—which means exhausted or worn out. It can also mean some piece of equipment which is damaged or broken. Both senses are common.

Where it comes from is not entirely certain. A knacker from the sixteenth century on was a harness maker or saddler. The word just might have come from knack, a trinket (which we still have, but only as one half of the reduplicated knick-knack), because the knacker originally only made the small bits of harness. Another sense from the beginning of the nineteenth century was for a person who bought old or worn-out horses and slaughtered them for their meat, hides and hoofs. He worked from a knacker’s yard. A possible link with the modern slang sense is obvious enough: if you’re knackered you’re fit only for the knacker’s yard.

But there’s another slang sense of knackers, for the testicles, which grew up a little later, possibly also from knack, but possibly from yet another sense of knacker, that of castanets (which could be an altered form of knockers, but might come from an obsolete sense of knack, to knock or to make a sharp, abrupt noise). To knacker, therefore, is to castrate.

Modern dictionaries are cautious about whether knackered has its origin in the horse-slaughterer sense or the castration one. However, British men often use it in such a way that they take it to mean the latter, even if that isn’t actually where it came from.