Definition of "guinea"

English, French, Spanish, Portuguese, Georges, and Louises, doubloons and double guineas and moidores and sequins, the pictures of all the kings of Europe for the last hundred years, strange Oriental pieces stamped with what looked like wisps of string or bits of spider's web, round pieces and square pieces, and pieces bored through the middle, as if to wear them round your neck—nearly every variety of money in the world must, I think, have found a place in that collection...

1881–1882, Robert Louis Stevenson, Treasure Island, London, Paris: Cassell & Company, published 14 November 1883

However, since there are 488 pages in all for a bargain price of a guinea one must not be too carping.

1962 June, “New Reading on Railways: Locomotives of British Railways, by H. C. Casserley & L. Asher, Spring Books, 21s.”, in Modern Railways, page 432

The guineas peeped complainingly, the goslings waddled into all the puddles and came back to chill my skin.

1944, Emily Carr, “Brooding and Homing”, in The House of All Sorts‎

If I’m to tell the whole truth—and why not? I sure have the time!—I’ll have to start by saying I was born Richard Pinzetti, in New York’s Little Italy. My father was an Old World guinea.

1982, Stephen King, Survivor Type