The phrase to eat the calf in the cow’s belly means to anticipate unduly, in particular, to spend one’s revenue before it comes in.
—Synonym: to count one’s chickens before they are hatched.

The phrase to eat the calf in the cow’s belly, though obsolete, is still occasionally used—as in the following from Albertopolis, right on cue, by David Mellor, published in The Guardian (London and Manchester, England) of Friday 18^th February 1994:

I am reluctant to join that happy band of people who are already spending the anticipated proceeds of the Lottery many months before they roll in—eating the calf in the cow’s belly we used to call it down in Dorset.

The earliest occurrences of the phrase to eat the calf in the cow’s belly that I have found are as follows, in chronological order:

1-: From The complete English Tradesman, in familiar Letters; Directing him in all the several Parts and Progressions of Trade (London: Printed for Charles Rivington, 1726), by the English novelist and journalist Daniel Defoe (1660-1731):

Trade is anticipated by Credit, and it grows by the anticipation; for men often buy cloaths before they pay for them, because they want cloaths before they can spare the money; and these are so many in number, that really they add a great stroke to the bulk of our Inland trade: How many families have we in England that live upon credit, even to the tune of two or three years rent of their revenue, before it comes in? so that they may be said to eat the Calf in the Cow’s belly: This encroachment they make up on the stock in trade; and even this very article may state the case: I doubt not but at this time the land owes to the trade some millions sterling; that is to say, the Gentlemen owe to the Tradesmen so much money, which at long run the rents of their lands must pay.

2-: From Gnomologia: Adagies and Proverbs; Wise Sentences and Witty Sayings, Ancient and Modern, Foreign and British (London: Printed for B. Barker, A. Bettesworth and C. Hitch, 1732), by the British physician Thomas Fuller (1654-1734):

He eats the Calf in the Cow’s Belly.

3-: From L’Avare. Comédie. Par Mons. de Molière. The Miser. A Comedy. New done into English from the French of Molière (London: Printed for B. Lintot: And sold by H. Lintot, 1732), by the French playwright Molière (Jean-Baptiste Poquelin – 1622-1673) and the English translator John Ozell (died 1743):

[Translator’s note:] Mangeant son Bled en herbe. […] I translate it […] The Corn in the Ear. […] To eat one’s Corn in the Ear, or in the Crop, is a French Proverb, signifying to spend one’s Rents before they come in: As we say, To eat the Calf in the Cow’s Belly.
[…]
[Translation of the French text*:] La Fleche. I see, Sir, under favour, you’re in the same high Road to Ruin as Panurgus, taking up Money beforehand, buying dear, selling cheap, and eating your Corn in the Ear. [The Calf in the Cow’s Belly, we say.]

* The original French text is as follows in L’Avare (Paris: Jean Ribou, 1669), by Molière:

La Fleche. Ie vous voy, Monsieur, ne vous en déplaise, dans le grand chemin justement que tenoit Panurge pour se ruiner, prenant argent d’auance, achetant cher, vendant a bon marché, & mangeant son bled en herbe.

4-: From Clarissa. Or, The History of a Young Lady. Comprehending the most Important Concerns of Private Life. And particularly shewing, The Distresses that may attend the Misconduct Both of Parents and Children, In Relation to Marriage (Dublin: Printed by George Faulkner, 1748), an epistolary novel by English author Samuel Richardson (1689-1761):

Nothing sooner brings down a proud spirit, than a sense of lying under pecuniary obligations. This has always made me solicitous to avoid laying myself under any such: Yet sometimes formerly have I been put to it, and cursed the tardy revolution of the quarterly periods. And yet I ever made shift to avoid anticipations: I never would eat the calf in the cow’s belly, as Lord M.’s phrase is.

5-: From The Idioms of the French and English Languages (London: Printed for J. Nourse, 1751), by Lewis Chambaud (died 1776):

Manger son blé en hèrbe [sic]. To eat one’s corn in the grass. To anticipate one’s revenue. To spend one’s revenue before it comes in. To eat the calf in the cow’s belly.

6-: From A new French Dictionary (London: Printed for J. Nourse and P. Vaillant, 1771), by Thomas Deletanville:

he eats the calf in the cow’s belly, il mange son bled en herbe.

7-: From The Administration of the British Colonies (London: Printed for J. Walter, 1774), by Thomas Pownall (1722-1805), Lieutenant-Governor of New Jersey from 1755 to 1757, and Governor of Massachusetts Bay from 1757 to 1760:

So far as our Colony-trade is a monopoly, government should precisely direct its care to two essential points: 1st, That all the profits of the culture, produce and trade of these Colonies, center finally in the mother country.—2dly, That the Colonies continue to be the customers purchasing of the mother country alone. On the other hand, so far as their circuitous trade either extends the commerce of the mother country, or creates by its activity a balance which finally centers in it—every relaxation, which is not destructive of this monopoly, should, if not of favour, yet of worldly prudence, be permitted to these Colonies. Further, as this monopoly is an oppressive, though a necessary abridgement of that freedom, which British subjects within the realm enjoy, and is an imposition, if not a direct tax, to the amount of the external balance of such trade—wise and true policy will be careful how (to use the expression of an old proverb) it “taxes the calf in the cow’s belly.”