In her book Toward Zero, author Agatha Christie has the following dialogue:
Kay said: ‘I don’t like my colour scheme in the livingroom. Can I have it done over, Nevile?’
‘Anything you like, beautiful.’
‘Peacock blue,’ said Kay dreamily, ‘and ivory satin cushions.’
‘You’ll have to throw in an ape,’ said Nevile.
‘You can be the ape,’ said Kay.
What on earth is that supposed to mean, "throw in an ape"? I have no idea. Is that an expression?
It's not an expression. It's a reference to a poem Cargoes by John Masefield. It was extremely popular in Agatha Christie's time, and most of her readers would be familiar with it.
The first stanza goes:
Quinquireme of Nineveh from distant Ophir,
Rowing home to haven in sunny Palestine,
With a cargo of ivory,
And apes and peacocks,
Sandalwood, cedarwood, and sweet white wine
Kay has asked for peacock and ivory to be in her decor, and Nevile says she will have to "throw in" (i.e. include) an ape to complete the set.
The poem itself references a Bible passage (1 Kings 10:22 and 2 Chronicles 9:21)
Once in three years came the navy of Tharshish, bringing gold, and silver, ivory, and apes, and peacocks.
DJClayworth’s answer explains the humurous allusion in the line, but it’s worth also explaining the idiomatic meaning of the sentence (independently of the allusion), which would be clear to most native British speakers but may not be obvious to non-natives.
In many contexts, especially negotiation/bargaining, to throw in something means to add it to the offer under discussion. “This bicycle is a hundred dollars.” “Too expensive, it’s old and rusty.” “I’ll throw in the lock and lights as well.” “OK, it’s a deal.” In the linked quote, Kay and Nevile are discussing redecorating the room, and Kay has described what she wants in the new decoration. In that context, Nevile’s “You’ll have to throw in an ape” means “you should add an ape to your proposed decorations”.
