Often in the phrase let's get cracking. The entry for "get cracking" in the original edition of Eric Partridge, Dictionary of Slang and Unconventional English (1936) is actually a bit more detailed than Lighter's citation suggests: get cracking. To begin work : Royal Air Force : from ca. 1925. I.e. cracking on speed. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.