Alluding to the possibility of finding a pearl in an oyster, the phrase the world is one’s oyster means that one is in a position to profit from the opportunities that life, or a particular situation, may offer.

It was coined by the English poet and playwright William Shakespeare (1564-1616) in The Merry Wiues of Windsor (Folio 1, 1623). Nym and Pistol, two of Falstaff’s men, have refused to convey the latter’s letters to Mistresses Ford and Page, saying that they want to behave respectably:

Falstaff: I will not lend thee a penny.
Pistol: Why then the world’s mine Oyster, which I,
with sword will open.

Pistol is poor because he insists on maintaining some honour. According to B. A. Phythian in A Concise Dictionary of Phrase and Fable (1993), in this scene Pistol means that he will use his sword to extract money from an unwilling world, a sense removed from the modern phrase which means that the world is simply waiting to be opened up to provide good things.

Shakespeare might have been alluding to a proverb that the Church of England clergyman Thomas Fuller (1608-61) mentioned and explained in The History of the Worthies of England (London, 1662):

The Mayor of Northampton opens Oysters with his Dagger.
This Town being 80 miles from the Sea, Sea fish may be presumed stale therein.

By opening oysters with his dagger, the mayor of Northampton keeps them at a sufficient distance from his nose. Phythian explains that Pistol’s proposed use of his sword to effect the opening―a comically cumbersome operation―may imply an even greater degree of rottenness in the oyster, that is to say in the world. This colouring too is absent from the modern use of the expression.

The phrase seems to have lain dormant for two centuries after Shakespeare’s time. It reappeared in The Adventures of Gil Blas of Santillane (London, 1809), the translation by the British author Benjamin Heath Malkin (1769-1842) of Histoire de Gil Blas de Santillane (1715 to 1735), a picaresque novel by the French playwright and novelist Alain-René Lesage (1668-1747):

He wants a licence to sell his drugs during the term of ten years in all the towns of the Spanish monarchy, to the exclusion of all other quacks; in short, a monopoly of poisons. In gratitude for this patent to thin mankind, he will present the donor with a gratuity of two hundred pistoles. I looked superciliously, like a patron, at the mountebank, and told him that his business should be done. As lameness and leprosy would have it, in the course of a few days, I sent him on his progress through Spain, invested with full powers to make the world his oyster, and leave nothing but the shell to his unpatented competitors.

The image is absent from the original French text, where tromper le peuple simply means to deceive the people, and exclusivement refers to the exclusive rights to do so:

(1750 edition)
Il demande un privilege pour débiter ses drogues, pendant l’espace de dix années, dans toutes les villes de la monarchie d’Espagne, à l’exclusion de tous autres ; c’est-à-dire qu’il soit défendu aux personnes de sa profession de s’établir dans les lieux où il sera. Par reconnoissance il comptera deux cents pistoles à celui qui lui remettra le dit privilege expédié. Je dis au Saltimbanque, en tranchant du protecteur : Allez, mon ami, je ferai votre affaire. Véritablement, peu de jours après, je le renvoyai avec des patentes qui lui permettoient de tromper le peuple exclusivement dans tous les royaumes d’Espagne.