Hyrcania was a country spanning parts of what is now Iran and Turkmenistan, covering the majority of the low wooded plains along the southern shores of the Caspian Sea. It existed under the rule of various empires from at least 500 BCE to 800 CE, and was well-known in the classical world -- in fact, the Greeks called the Caspian Sea the 'Hyrcanian Sea'.
Hyrcania was well-known as being full of game animals of all sorts; in fact, Hyrcania literally means 'Wolf-land'. One could hunt, or be hunted by, vicious wolves, wild boar, and panthers. And, you guessed it, tigers. These were what we today would call the Caspian tiger, although they are now extinct. Despite their reputation, the Caspian/Hyrcanian tiger was in fact smaller, on average, than the Siberian tiger. The last tiger in Hyrcania was probably killed sometime in the 1970s, although Caspian tigers were reported in the wild in Turkey until the mid-80s.
Hyrcania's reputation for producing the most fearsome of beasts, and especially tigers, was widespread. This reputation lasted long after the political unity of Hyrcania has ended, to Shakespeare's day and beyond.
Re-enter GHOST OF BANQUO
"sight! let the earth hide thee!
Thy bones are marrowless, thy blood is cold;
Thou hast no speculation in those eyes
Which thou dost glare with!"
"What man dare, I dare:
Approach thou like the rugged Russian bear,
The arm'd rhinoceros, or the Hyrcan tiger;
Take any shape but that, and my firm nerves
Shall never tremble: or be alive again,
And dare me to the desert with thy sword;
If trembling I inhabit then, protest me
The baby of a girl. Hence, horrible shadow!
Unreal mockery, hence!
--Macbeth by William Shakespeare; Macbeth complains to ghost that it is too scary.
While Hyrcanian tigers were well known as being fierce and vicious, there was one thing even more heartless -- a man who had been raised by one of these beasts. 'To have been suckled by a Hyrcanian tiger' meant that you were uncaring and unfeeling.
"Thus while he spoke, already she began, With sparkling eyes, to view the guilty man; From head to foot survey'd his person o'er, Nor longer these outrageous threats forebore: "False as thou art, and, more than false, forsworn! Not sprung from noble blood, nor goddess-born, But hewn from harden'd entrails of a rock! And rough Hyrcanian tigers gave thee suck! Why should I fawn? what have I worse to fear? Did he once look, or lent a list'ning ear, Sigh'd when I sobb'd, or shed one kindly tear?"1
-- Aeneid by Virgil; Book IV; Dido accuses Aeneas.
This single passage of the Aeneid may well be the primary reason for the use of the phrase in this sense. Shakespeare had read some Virgil, and makes other references to his works. Other poets and authors also borrow heavily from Virgil, with little disguise, and those who are not borrowing from Virgil are borrowing second hand from Shakespeare.
One famous rip-off of Virgil's passage is Italian poet Torquato Tasso's epic poem Jerusalem Delivered (La Gerusalemme liberata) (1581):
"Né te Sofia produsse e non sei nato
de l’azio sangue tu; te l’onda insana
del mar produsse e ’l Caucaso gelato,
e le mamme allattàr di tigre ircana.
Che dissimulo io piú? l’uomo spietato
pur un segno non diè di mente umana.
Forse cambiò color? forse al mio duolo
bagnò almen gli occhi o sparse un sospir solo?"
"Sophia did not give birth to you, and you are
not born of the blood of the Azzi; the raging
ocean wave frozen Caucasus gave birth to you,
and the dugs of the Hyrcanian tigress gave you
milk. Why do I pretend any longer? The heartless
man gave not a single sign of human emotion. Did
he change color, perhaps? Did he at least for my
sorrow bathe his eyes, perhaps, or drop a single
Verse 57, Translation by Lawrence F. Rhu.
Although more importantly, at least in the English speaking-world, Shakespeare himself refers twice more to the Hyrcanian tiger, once in Hamlet II.ii, in which he refers specifically to the passage in the Aeneid:
"...One speech in it I
chiefly loved: 'twas Aeneas' tale to Dido; and
thereabout of it especially, where he speaks of
Priam's slaughter: if it live in your memory, begin
at this line: let me see, let me see–
'The rugged Pyrrhus, like the Hyrcanian beast,'–"
And again in Henry VI, Part 3 I.iv, as the Duke of York bemoans the actions of Queen Margaret:
"That face of his the hungry cannibals
Would not have touched, would not have stained with blood:
But you are more inhuman, more inexorable,
O, ten times more, than tigers of Hyrcania.
See, ruthless queen, a hapless father's tears:
This cloth thou dippedst in blood of my sweet boy,
And I with tears do wash the blood away.
Keep thou the napkin, and go boast of this:
And if thou tell'st the heavy story right,
Upon my soul, the hearers will shed tears;
Yea, even my foes will shed fast-falling tears,
And say “Alas, it was a piteous deed!”
Despite all of this, references to the Hyrcanian tiger have died out except in the most literary of contexts, and most people will not recognize the reference. The older "raised by wolves" has won the feral child contest, and newcomer heart of stone (c.1200s) has taken its place as the idiom of choice for 'heartless'.