Dulce et Decorum est - Wilfred Owen, out of World War 1:
Ah well. In that case, if you’re going for a small body part, you might want to consider:
Molon labe
Google for full details!
How about this excerpt from Milton’s “Paradise Lost”? Satan’s speech to rally his troops after they just got booted out of Heaven is pretty badass:
Innumerable force of Spirits armed,
That durst dislike his reign, and, me preferring,
His utmost power with adverse power opposed
In dubious battle on the plains of Heaven
And shook his throne. What though the field be lost?
All is not lost—the unconquerable will,
And study of revenge, immortal hate,
And courage never to submit or yield:
And what is else not to be overcome?
I’ve always been partial to
“Incident of the French Camp,” Robert Browning:
From The Rehearsal, by George Villiers, the 2nd Duke of Buckingham:
Since I know something about President’s I know this is Abe Lincoln’s favorite poem, and it is a Masterpiece, Oh why should the spirit of mortal be proud, in part and more like it;
The saint who enjoyed the communion of heaven;
The sinner who dared to remain unforgiven;
The wise and the foolish, the guilty and just, Have quietly mingled their bones in the dust.
Which reminds me of Omar Khayyam (via Edward FitzGerald):
Why, all the Saints and Sages who discuss’d
Of the Two Worlds so wisely–they are thrust
Like foolish Prophets forth; their Words to Scorn
Are scatter’d, and their Mouths are stopt with Dust.
Yep.
“One two, one two, and through and through,
His vorpal blade went snickersnack,
He left it dead, and with its head,
He went galumphing back.”
That’s pretty bad-ass. Except maybe for the “galumphing”. I’m really not sure how bad-ass “galumphing” is.
Then you really can’t beat Psalm 18:37-45 for biblical badassery:
Others have already beaten me to my first two choices (Ballad of East and West by Kipling and The Highwayman by Noyes–which, btw, has a great musical version by Loreena McKennitt) but how about this one, for certain values of “badass”:
Down, Wanton, Down
by Robert Graves
Down, wanton, down! Have you no shame
That at the whisper of Love's name,
Or Beauty's, presto! up you raise
Your angry head and stand at gaze?
Poor bombard-captain, sworn to reach
The ravelin and effect a breach--
Indifferent what you storm or why,
So be that in the breach you die!
Love may be blind, but Love at least
Knows what is man and what mere beast;
Or Beauty wayward, but requires
More delicacy from her squires.
Tell me, my witless, whose one boast
Could be your staunchness at the post,
When were you made a man of parts
To think fine and profess the arts?
Will many-gifted Beauty come
Bowing to your bald rule of thumb,
Or Love swear loyalty to your crown?
Be gone, have done! Down, wanton, down!
It’s got to be at least a little bit badass to write a poem about your wang… 
I’m fond of"Old Ironsides",written by Oliver Wendell Holmes, Sr. to protest the decommissioning of the USS Constitution.
Aye tear her tattered ensign down
long has it waved on high,
And many an eye has danced to see
That banner in the sky;
Beneath it rung the battle shout,
And burst the cannon’s roar;–
The meteor of the ocean air
Shall sweep the clouds no more.
Her deck, once red with heroes’ blood,
Where knelt the vanquished foe,
When winds were hurrying o’er the flood,
And waves were white below,
No more shall feel the victor’s tread,
Or know the conquered knee;–
The harpies of the shore shall pluck
The eagle of the sea!
Oh, better that her shattered hulk
Should sink beneath the wave;
Her thunders shook the mighty deep,
And there should be her grave;
Nail to the mast her holy flag,
Set every threadbare sail,
And give her to the god of storms,
The lightning and the gale!’
Thanks for that. A couple of references from MAS*H (the novel) just dropped into place after the thick end of forty year. 
T. S. Eliot from The Wasteland:
And I will show you something different from either
Your shadow at morning striding behind you
Or your shadow at evening rising to meet you;
I will show you fear in a handful of dust.
It’s the last line there that gets me.
Virgil’s Aeneid:
Arma virumque cano, Troiae qui primus ab oris
Italiam, fato profugus, Laviniaque venit
litora, multum ille et terris iactatus et alto
vi superum saevae memorem Iunonis ob iram;
multa quoque et bello passus, dum conderet urbem,
inferretque deos Latio, genus unde Latinum,
Albanique patres, atque altae moenia Romae.
But for a tattoo, I’d suggest short and sweet, maybe a haiku?
Kipling pulled strings to get his son John into the army after he had been twice rejected as medically unfit. John was killed shortly after being sent to the front, his body never found. After the war, Kipling wrote,
“If any question why we died,
Tell them, because our fathers lied.”
A.E. Housman’s Epitaph on an Army of Mercenaries:
These, in the day when heaven was falling,
The hour when earth’s foundations fled,
Followed their mercenary calling,
And took their wages, and are dead.
Their shoulders held the sky suspended;
They stood, and earth’s foundations stay;
What God abandoned, these defended,
And saved the sum of things for pay.
The first poem I ever memorized. Also set to music!
And there’s his untitled “III” from Last Poems:
Her strong enchantments failing,
Her towers of fear in wreck,
Her limbecks dried of poisons
And the knife at her neck,
The Queen of air and darkness
Begins to shrill and cry,
'O young man, O my slayer,
To-morrow you shall die.'
O Queen of air and darkness,
I think 'tis truth you say,
And I shall die to-morrow;
But you will die to-day.
William Blake - The Tyger
Tyger Tyger, burning bright,
In the forests of the night;
What immortal hand or eye,
Could frame thy fearful symmetry?
I always liked this one by Henry Wadsworth Longfellow
Tell me not, in mournful numbers,
Life is but an empty dream!?
For the soul is dead that slumbers,
and things are not what they seem.
Life is real! Life is earnest!
And the grave is not its goal;
Dust thou art; to dust returnest,
Was not spoken of the soul.
And this one I use to remind myself that things are seldom what they seem (although it might be a bit long for a tattoo.
Richard Cory
Whenever Richard Cory went down town,
We people on the pavement looked at him:
He was a gentleman from sole to crown,
Clean favored, and imperially slim.
And he was always quietly arrayed,
And he was always human when he talked;
But still he fluttered pulses when he said,
‘Good-morning,’ and he glittered when he walked.
And he was rich - yes, richer than a king -
And admirably schooled in every grace:
In fine, we thought that he was everything
To make us wish that we were in his place.
So on we worked, and waited for the light,
And went without the meat, and cursed the bread;
And Richard Cory, one calm summer night,
Went home and put a bullet through his head.
Edwin Arlington Robinson
This from Housman would make an awesome tattoo: