The 50 Rupee Seychelles note is due to be sold at Duke's of Dorchester in October.
It was issued between 1968 and 1973 and features an image of the Queen. It has an estimate of £200.
Timothy Medhurst, of Duke's, said it is not known for sure who left the hidden message in the note, but it could have been done by the printers.
He said some believe the word was added as the Seychelles was trying to gain independence from Britain.
Another note in the series contains the word "scum", suggesting the additions were deliberate.
Fifty Rupees is worth about £2.50 ($3.80).
Mr Medhurst said: "This is a very collectable item for obvious reasons.
"Amazingly the added message wasn't noticed, or was perhaps never mentioned, until after it stopped being printed.
"Many think the engraver Brian Fox of Bradbury & Wilkinson, the printers, put it in.
"It is an otherwise attractive note but would have disappeared into obscurity if it had not been for its secret message."
A collection of coins, notes and medals is to be auctioned on 15 October.
A banknote with the word "sex" secretly written in palm trees is to be sold at auction.