xy

Source: The Adventures of Tom Sawyer & Adventures of Huckleberry Finn

Source: The Adventures of Huckleberry Finn

Often attributed to Twain online, but unsourced. Alternate source: "The whole principle [of censorship] is wrong. It's like demanding that grown men live on skim milk because the baby can't have steak." — Robert Heinlein, The Man Who Sold the Moon, 1951, p. 188.
Misattributed

Pudd'nhead Wilson's New Calendar, Ch. V
Following the Equator (1897)

Source: The Prince and the Pauper

Pudd'nhead Wilson's New Calendar, Ch. XX
Following the Equator (1897)

Part VI: "Two Fragments from a Suppressed Book Called 'Glances at History' or 'Outlines of History' ".
Papers of the Adams Family (1939)
Context: Against our traditions we are now entering upon an unjust and trivial war, a war against a helpless people, and for a base object — robbery. At first our citizens spoke out against this thing, by an impulse natural to their training. Today they have turned, and their voice is the other way. What caused the change? Merely a politician's trick — a high-sounding phrase, a blood-stirring phrase which turned their uncritical heads: Our Country, right or wrong! An empty phrase, a silly phrase. It was shouted by every newspaper, it was thundered from the pulpit, the Superintendent of Public Instruction placarded it in every schoolhouse in the land, the War Department inscribed it upon the flag. And every man who failed to shout it or who was silent, was proclaimed a traitor — none but those others were patriots. To be a patriot, one had to say, and keep on saying, "Our Country, right or wrong," and urge on the little war. Have you not perceived that that phrase is an insult to the nation?
For in a republic, who is "the Country"? Is it the Government which is for the moment in the saddle? Why, the Government is merely a servant — merely a temporary servant; it cannot be its prerogative to determine what is right and what is wrong, and decide who is a patriot and who isn't. Its function is to obey orders, not originate them. Who, then, is "the country?" Is it the newspaper? Is it the pulpit? Is it the school-superintendent? Why, these are mere parts of the country, not the whole of it; they have not command, they have only their little share in the command. They are but one in the thousand; it is in the thousand that command is lodged; they must determine what is right and what is wrong; they must decide who is a patriot and who isn’t.

A Tramp Abroad (1880)
Context: You may say a cat uses good grammar. Well, a cat does -- but you let a cat get excited once; you let a cat get to pulling fur with another cat on a shed, nights, and you'll hear grammar that will give you the lockjaw. Ignorant people think it's the noise which fighting cats make that is so aggravating, but it ain't so; it's the sickening grammar they use.

Source: What Is Man? (1906), Ch. 6