The term monkey business, which alludes to the proverbial playfulness of monkeys, means mischievous or deceitful behaviour.
According to the Oxford English Dictionary (3rd edition, 2002), this term is probably modelled on Bengali bãdrāmi; this dictionary refers to modern Sanskrit vānara-karman, from vānara, monkey, and karman, action, work, employment, and to Hindi vānara-karma.
The earliest instance of monkey business that I have found is from The Examiner (London) of Sunday 15th February 1835:
An article on Spanish history [see note 1], in the forthcoming Westminister [sic] Review for April, concludes with the following intimation to the moderates, the waverers, and the turncoats, at the present crisis:—
“The inference from all this, is only a brick from the mass of evidence which history has piled up all over the world, of the worthlessness, wretchedness, and contemptibleness of monarchical government, wherever it has not been diluted by the control of popular power, to an extent that left little more for a prerogative to do than
‘—in his cage, like parrot fine and gay,
Be kept to strut, look big, and talk away.’ [see note 2]
The English Tories have insisted upon stirring up these questions, after everybody thought they had dropped by a kind of common consent. If there was once a Stuart prerogative in England, there was a splendid Republic too; and the fools that re-hoisted the one at the fore, by the same act run up the other at the mizen. It remains for the juste milieu, which are amidships, to consider whether they will not quash this ‘monkey business,’ by hauling down the rag of offence, on condition that the other shall disappear along with it.”
The second-earliest instance that I have found is from a letter by Thomas Perronet Thompson (1783-1869), British Parliamentarian, published in The Constitutional; and Public Ledger (London) of Monday 24th April 1837—the author seemed to confirm that monkey business is a calque of an Indic term:
Suppose an assassin had decreed that no quarter should be given to the prisoners taken from their force, would they have stopped to hear us turn phrases (if we had been disposed to do it), on the humanity of the said assassin in having dismissed six prisoners on the ground of their being not fighting men but hired musicians, manifestly with a view to making (according to another of the expressions which I have heretofore found puzzling) one of those exceptions which confirm the rule, and shooting all he took in arms? They would have committed themselves to no such “monkey business” as the Indians call it, before mankind; they would have seen the Muse of History in the shape of a printer’s devil, dipping a broad sheet into the abysses of a steam-press, and consigning them to “everlasting redemption” in the memory of all that is to come.
1-: The quotation from The Westminster Review (London) of April 1835 is in fact from the review of Memoirs of Spain, during the Reigns of Philip IV. and Charles II. From 1621 to 1700, by John Colin Dunlop (1785-1842), Scottish advocate and historian.
2: The reference is to the following lines from Table Talk, by William Cowper (1731-1800), English poet and hymnodist:
Thus proud Prerogative, not much revered,
Is seldom felt, though sometimes seen and heard;
And in his cage, like parrot fine and gay,
Is kept to strut, look big, and talk away.