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”.
The scientific name for the bone is the furcula, from the Latin word that is a diminutive of furca, a fork. That is actually also the origin of our fork and also turns up in words like bifurcated, which the furcula, of course, is.