Where does the phrase "get crackin'" come from?

I always though this was related to the cracking of a whip; either to make a horse run faster or to drive cattle etc.

I think the precise origin will probably always be shrouded in mystery, but this Ngrams graph implies it derives from the earlier British usage Crack on.

I realise Ngrams will have included many spurious occurrences of both phrases (for example, references to a crack on a surface). But this 1764 usage is obviously idiomatic, so we can safely say Crack on was current by then. I can't find any explicit use of Get cracking until at least a hundred years later.

I suggest the basic metaphor being invoked is that in many contexts, a crack is the first stage of a wider split; starting a major task might imply you need to make a small dent or crack in it first. Also, of course, you can't make an omelette without breaking (cracking) eggs.

Get cracking may have originally been not much more than get to talking as seen in this clip from Passages in the Life of Mrs. Margaret Maitland, 1849 (check)

With John Boag's 1848 Popular and Complete English Dictionary having one definition of cracking to be "conversing familiarly," the last sentence of the above clip can be translated, "The children of that time couldn't have sat chatting about this around the fire."

This use of cracking is related to the phrase to crack a joke, but I'm not sure it's related to the "get to work" meaning of get cracking. Just thought this early appearance was interesting. For more theories on the modern meaning check out the thread on the phrase at Wordwizard.

After looking at the earliest google n-grams of usage of the phrase, (discounting the 1849 passage which clearly uses "cracking" in the sense of talking/gossiping/boasting) I had a hunch it had its origins in military jargon. A little more searching found this:

According to the Dictionary of Slang and Unconventional English:

get cracking-get mobile-get skates on-get stuck into it-get weaving. To respond (immediately to an order; to get a move on; Services’ (the 2nd, 3rd, 4th, general; the 1st orig. Army and then gen.; the 5th, RAF—see separately at weaving): since ca. 1925, except stuck (ca. 1916) and the last q.v. All usu. in the imperative. Origins: whip-cracking at the mustering of cattle; …