The noun Greek has long been used in the sense of unintelligible speech or language, gibberish, and the phrase it’s (all) Greek to me means I can’t understand it at all.

This expression is well known from The Tragedie of Julius Cæsar (1599), by the English playwright and poet William Shakespeare (1564-1616):

(Folio 1, 1623)
Cassius. Did Cicero say any thing?
Casca. I, he spoke Greeke.
Cassius. To what effect?
Casca. Nay, and I tell you that, Ile ne’re looke you
i’th’face againe. But those that vnderstood him, smil’d
at one another, and shooke their heads: but for mine
owne part, it was Greeke to me.

Casca is a blunt character, and his remark is probably no more than a sardonic dismissal of Cicero’s scholarship and subtlety. But the more discerning members of Shakespeare’s audience would have known that the Roman statesman, orator and author Cicero (106-43 BC) was a Greek scholar and had studied the language at Athens. He was devoted to Greek literature and philosophy, to the extent that he was nicknamed the Grecian according to the Greek biographer and philosopher Plutarch (circa 46-circa 120), who was Shakespeare’s source.

Shakespeare popularised a phrase that already existed. In particular, Tis Greeke to mee is found in The Scottish historie of Iames the fourth, slaine at Flodden (1598), by the English author Robert Greene (1558-92).

But the phrase seems to have been first used by the English poet George Gascoigne (circa 1539-1577). He translated an Italian comedy written by Ludovico Ariosto (1474-1533), I Suppositi (1509 in prose, and later versified), under the title Supposes, and presented it at Grey’s Inn in 1566. At the very beginning of the play, Balia, the nurse, speaks with Polynesta, the young woman. Balia, after asking Polynesta

Then I understande you not: how sayde you?

says

This geare [= talk] is Greeke to me; either it hangs not well togither, or I am very dull of understanding: speak plaine, I pray you.

The expression is absent from the Italian text. In the original version, after asking

Che di’ tu dunque?

Balia says

O questo non può stare insieme, o intendere
Io non ti debbo, sì che meglio esprimelo.

Whether the English expression was coined by Gascoigne or was a contemporary idiom is not known.

(Interestingly, Shakespeare is known to have looked closely at Gascoigne’s Supposes. The influence of this comedy of subterfuge and misunderstanding can be seen in particular in The Taming of the Shrew and The Comedy of Errors.)

What is probable in any case is that the English phrase is derived from medieval Latin Graecum est, non potest legi, it is Greek, it cannot be read.

A variant, heathen Greek, was used for example by the English preacher and cofounder of Methodism John Wesley (1703-91) on 10^th October 1745 in Advice to a People called Methodist:

(1841 edition)
Perhaps not one in a hundred of those who use the term Methodist have any ideas of what it means. To ninety-nine of them it is still heathen Greek. Only they think it means something very bad,—either a Papist, a heretic, an underminer of the Church, or some unheard-of monster.

Another variant, St. Giles’s Greek, was first defined by the English antiquary and lexicographer Francis Grose (1731-91) in A Classical Dictionary of the Vulgar Tongue (1^st edition – 1785):

– Gibberish, cant language, pedlars French; or St. Giles’s Greek. See St. Giles’s Greek.
– Greek, St. Giles’s Greek, the slang lingo, cant, or gibberish.

Grose explained why this particular cant came to be named after the parish of St. Giles:

– Gile’s or St. Giles’s breed, fat, ragged, and saucy; Newton and Dyot Streets, the grand head-quarters of most of the thieves and pickpockets about London, are in St. Giles’s parish.

The author also gave some insights into life in St. Giles:

– Hopping Giles, a jeering appellation given to any person who limps, or is lame. St. Giles was the patron of cripples, lepers, &c. churches dedicated to that saint, commonly stand out of a town, many of them having been chapels to hospitals.
– Bitch, a she dog, or dogess; the most offensive apellation [sic] that can be given to an English woman, even more provoking than that of whore, as may be gathered from the regular Billinsgate or St. Giles’s answers, “I may be a whore, but can’t be a bitch.”
– Cheeks, ask cheeks near cunnyborough¹, the repartee of a St. Giles’s fair one, who bids you ask her backside, anglice² her a—se.

¹ cunny: the female genitals
² anglice: in plain English.

Jackson’s Oxford Journal (Oxfordshire) of 4^th March 1786 published a letter purportedly written in this particular idiom:

We are desired to publish the following intercepted Letter to the Informer of the Robbery at Magdalen College, written, as it appears, by one of the Gang, in the Language which they call St. Giles’s Greek:
“I dont [sic] blame thee for turning stag³, for to be sure every man is in the right to take care of his own self: but what made thee tell so many lies to the Queer Cuffin⁴? How couldst say that the golden stick at New College Chapel is stuck all with diamonds, and that there was a design to nab it, and make all our fortunes? when every body knows that tis but a thin hollow thing with bits of blue glass, not altogether worth ten pound. Thou hast the luck to squeak now, and so thy friends must go to quod⁵ and scour the cramp rings⁶, while thou livest at ease with the Harmanbeck⁷: but look to thyself; for tho’ thou hast scapt the chates⁸ for this bout, I may see thee a babe in the wood before tis over: and so the Ruffin cly thee⁹.”

³ stag: informer
⁴ the queer cuffin: the judge
⁵ go to quod: go to prison
⁶ to scour the cramp-rings: to wear the shackles
⁷ the harman-beck: the constable
⁸ thou has scapt the chates: you have escaped the gallows
⁹ the Ruffin cly thee: the Devil take you