Origin of the expression "pull your finger out"
I've heard that "pull your finger out" came from muzzle loaded gunnery. One of the team firing the gun would put his finger in the hole during loading to prevent embers being ejected from the hole. When loading was complete, he would be ordered to pull his finger out.
But I can't find anything confirming this. Any ideas?
I have been told it is originally British RAF slang, meaning "pull your finger out of your ass". I believe it refers to the fact that if you have your finger 'up your ass', you can't be doing what you are supposed to be doing, so should remove your finger and begin to act. I would say it is synonymous with "stop procrastinating!"
It appears there is no agreement on the origin of this expression. Here is what I have found:
Pull your finger out:
When you tell someone to pull your finger out, you are telling them to hurry up or to complete a task quicker.
This term derives from when artillerymen would push gunpowder into the ignition chamber of their guns with their fingers and hold the powder in place by keeping their finger in the hole of the ignition chamber
(Normally, a wooden plug would be used for this purpose, but it was quicker to use a finger.) Keen to fire the weapon, the gun commander would shout pull your finger out to the powder filler just before the gun was fired.
The following source offers two possible versions of the origin:
According to "A Dictionary of Catch Phrases American and British," by Eric Partridge, revised by Paul Beale, "take (or pull or get) your finger out" originated about 1930 in the Royal Air Force and was adopted in 1941 or 1942 by the British army. The first edition of Partridge's book had the meaning as "Stop scratching your backside and get on with the job." The revised edition, having been enriched by further scholarship, offers a different meaning as the accurate one. It has to do with couples rather than individuals.
To pull your finger out is to hurry, to get a move on. This is another nautical saying and comes from the times of the Men'o'War. When the cannon were loaded a small amount of powder was poured into the ignition hole near the base of the weapon. In order to keep the powder secure before firing, a crew member pushed a finger into the hole. When the time came for ignition, the crewman was told to pull his finger out
Source:http://www.grammar-monster.com/sayings_proverbs/pull_your_finger_out.htm
Source: http://www.answerbag.com/q_view/341797
While there seems little agreement on the source of the phrase, n-gram and Google Books would seem to provide a certain amount of reason to eliminate most claims. https://books.google.com/ngrams/graph?content=pull+your+finger+out&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Cpull%20your%20finger%20out%3B%2Cc0 
gives a first usage of ~1875, which is far too late to support the naval origin. Starting in 1862, the Royal Navy began converting to breech-loaders (which obviously render the stated technique irrelevant), and by 1890 to 1900 naval artillery had universally switched to smokeless powder.
Examination of the Google Books excerpts associated with the phrase show no examples of the meaning "hurry up" prior to 1942, in agreement with the RAF slang origin.