(British English) the wishbone or furcula of a fowl, the forked bone between the neck and breast of a bird.
Again, all birds that can fly possess a 'merrythought', or furculum; and such is not found in the Pterodactyl. (H N Hutchinson, Extinct Monsters)
They had a chicken and a dish of ham between them, and he was feeding her with the merrythought... (Hugh Walpole, Judith Paris)
My uncle was in high good- humour, and especially always joking with Nora and the captain. It was, "Nora, divide that merrythought with the captain! see who'll be married first." (W M Thackeray, The Memoirs of Barry Lyndon)
Bless my drumsticks and merrythought, I shant be so cold and hungry, please God, this time to–morrow night. (Richard Doddridge Blackmore, Cradock Nowell: A Tale of the New Forest)
'wishbone of a fowl's breast,' c. 1600, from merry + thought. So called from the sport of breaking it between two persons pulling each on an end to determine who will get a wish he made for the occasion (the winner getting the longer fragment). (Online Etymology Dictionary)
It's the forked bone between the neck and breast of a bird. But that's the wishbone, I almost hear you cry. Indeed it is, but merrythought is the older term for that part of a turkey, chicken or other fowl served at table.
Wishbone was created in America; from the evidence, it seems to have appeared sometime around the 1850s, but has since taken over everywhere. But merrythought was still the more common term in America and Britain until about 1900. Here's an American example, from Mrs Goodfellow’s Cookery as it Should Be, published in Philadelphia in 1865:
Remove the merrythought and neck bones next, this you will accomplish by inserting the knife and forcing it under the bones, raise it and it will readily separate from the breast.
The name of wishbone comes, of course, from the folk custom in which two people hold its ends and pull, the one left with the longer piece making a wish. Merrythought refers to an older version of the custom, in which it is assumed that the one left with the longer piece will get to marry first. So the bone-pulling ceremony resulted in what were euphemistically called 'merry thoughts' among those taking part. This explains the reference in Jack Hinton, the Guardsman, an 1843 novel by the Irish writer Charles Lever: "Simpering old maids cracked merry thoughts with gay bachelors". (World Wide Words)