For example, it turns out that "its Greek to me" is a throw-away line from The Tragedy of Julius Caesar by Wm. Shakespeare in which a fellow conspirator tells Brutus what happened at the Senate that day. Someone was speaking from Greece. What did he say, asked Brutus. I have no idea, said the conspirator, it was Greek to me.
Here is a stanza from the poem in question, called "Gentlemen Rankers"
We have done with Hope and Honour, we are lost to Love and Truth,
We are dropping down the ladder rung by rung,
And the measure of our torment is the measure of our youth.
God help us, for we knew the worst too young!
Our shame is clean repentance for the crime that brought the sentence,
Our pride it is to know no spur of pride,
And the Curse of Reuben holds us till an alien turf enfolds us
And we die, and none can tell Them where we died.
And yet the language of his poems seem much more archaic, or at least filled with unrecognizable idiom, then your average late 19th century essay or poem. For example, Edgar Allen Poe's The Raven was first published in 1845, or 20 years before Kipling was even born, and yet The Raven is very readable today with very few archaic uses that are a problem. Well, as they say, the US and England are separated by a common language, and apparently this is even more so when you use a lot of idiom and slang.
A "Gentleman-Ranker" is a soldier in the British Army who is from the upper classes but finds himself an enlisted soldier (e.g. below his station in life). This would happen because of misfortune, a mistake, or a flaw in his character. But in any case, he has the education and manners of a member of the ruling class, but he is living the life of a common soldier. Hence, a "gentleman" who is a "ranker".
Other idiom in this poem which are still in common use include: something less than kind, black sheep, troop, thrash, down the ladder.
When researching this I came across the following image of Mickey, Donald and Pluto as the Three Musketeers, but some Internet wit had them labeled as "Gentlemen-Rankers", fallen from the upper classes to a mere soldier, but still showing here a certain spirit and elan.