“The God excuse, the last refuge of a man with no answers and no argument.”

April 7, 1775, p. 253
Boswell's full mention of this statement reads:
:Patriotism having become one of our topicks, Johnson suddenly uttered, in a strong determined tone, an apophthegm, at which many will start: "Patriotism is the last refuge of a scoundrel." But let it be considered, that he did not mean a real and generous love of our country, but that pretended patriotism which so many, in all ages and countries, have made a cloak of self-interest.
Life of Samuel Johnson (1791), Vol II

Phrases and Philosophies for the Use of the Young (1894)
Variant: Ambition is the last refuge of the failure
Context: Ambition is the last refuge of the failure.

"The Relation of Dress to Art," The Pall Mall Gazette http://www.gutenberg.org/files/14062/14062-h/14062-h.htm (February 28, 1885)
reprinted in Aristotle at Afternoon Tea:The Rare Oscar Wilde (1991)
Source: A Long Way Down

" O, Synecdoche, my Synecdoche! http://blogs.suntimes.com/ebert/2008/11/o_synecdoche_my_synecdoche.html," (10 November 2008)
Context: I was instructed long ago by a wise editor, "If you understand something you can explain it so that almost anyone can understand it. If you don't, you won't be able to understand your own explanation." That is why 90% of academic film theory is bullshit. Jargon is the last refuge of the scoundrel.

Source: City of Bones

Source: City of Bones

Variant: Sedate ignorance is the last stage of deterioration.
Source: Meditations in Wall Street (1940), p. 135

The trial of Charles B. Reynolds for blasphemy (1887)

The Study of History (1895)
Context: Most of this, I suppose, is undisputed, and calls for no enlargement. But the weight of opinion is against me when I exhort you never to debase the moral currency or to lower the standard of rectitude, but to try others by the final maxim that governs your own lives, and to suffer no man and no cause to escape the undying penalty which history has the power to inflict on wrong. The plea in extenuation of guilt and mitigation of punishment is perpetual. At every step we are met by arguments which go to excuse, to palliate, to confound right and wrong, and reduce the just man to the level of the reprobate. The men who plot to baffle and resist us are, first of all, those who made history what it has become. They set up the principle that only a foolish Conservative judges the present time with the ideas of the past; that only a foolish Liberal judges the past with the ideas of the present.