MEANING

The words merrythought and wishbone designate the furcula, i.e. the forked bone between the neck and breast of a bird.

The former is first recorded in A Worlde of Wordes, Or Most copious, and exact Dictionarie in Italian and English (London, 1598), by the English lexicographer of Italian descent John Florio (circa 1553-1625). The latter is of American-English origin; the earliest instance that I have found is from The Sun (Baltimore, Maryland) of 10^th November 1842:

Thanksgiving Day in Massachusetts.—The Boston Evening Bulletin thus notices the approach of Thanksgiving Day in the old Bay State:—“This happy day is fast approaching—the turkies are gobbling and growing fat, the chickens are crowing and getting plump, and the oxen are eating corn and pumpkins, that the pilgrims of old Massachusetts may have a feast, and their sons and daughters may have a frolic with wish-bones and mince-pies! There is no day like it among the nations of the earth.”

ORIGIN OF MERRYTHOUGHT

The traditional explanation of merrythought is as follows in the first edition (1906) of the Oxford English Dictionary:

The name, like the synonym wish-bone, alludes to the playful custom of two persons pulling the furcula of a fowl until it breaks; according to the popular notion, the one who gets the longer (in some districts, the shorter) piece will either be married sooner than the other, or will gain the fulfilment of any wish he may form at the moment.

This traditional explanation is euphemistic; the English antiquary and biographer John Aubrey (1626-97) gave the actual origin in Remaines of Gentilisme and Judaisme:

(1881 edition)
’Tis common for two to breake the Merrythought of a chick-hen, or wood-cock, &c., the Anatomists call it Clavicula; ’tis called the merrythought, because when the fowle is opened, dissected, or carv’d, it resembles the pudenda of a woman.

In the following passage from North-ward hoe (London, 1607), by Thomas Dekker (circa 1572-1632) and John Webster (circa 1580-1634), merrythought has sexual connotations:

(context: Greenshield and his wife, Kate, are lodging in a house with Greenshield’s friend, Featherstone, with whom Kate is having a clandestine affair. She fools Greenshield by pretending to suffer from sleepwalking, which makes her go into Featherstone’s room – Squirrel is a servant.)

– Greenshield. Was there euer any walking spirit, like to my wife? what reason should there bee in nature for this? I will question some Phisition: nor heare neither: vdslife*, I would laugh if she were in Maister Fetherstones Chamber, shee would fright him, Maister Fetherstone, Maister Fetherstone.
– Within Fetherstone. Ha, how now who cals?
– Greenshield. Did you leaue your doore open last night?
– Fetherstone. I know not, I thinke my boy did.
– Greenshield. Gods light shee’s there then, will you know the iest, my wife hath her old tricks, Ile hold my life, my wife’s in your chamber, rise out of your bed, and see and you can feele her.
– Squirill. He will feele her I warrant you?
– Greenshield. Haue you her sir?
– Fetherstone. Not yet sir, shees here sir.
Enter Fetherstone and Kate in his armes.
– Greenshield. So I said euen now to my selfe before God la: take her vp in your armes, and bring her hether softly, for feare of waking her: I neuer knew the like of this before God la, alas poore Kate, looke before God; shees a sleepe with her eyes open: prittie little roague, Ile wake her, and make her ashamd of it.
– Fetherstone. O youle make her sicker then.
– Greenshield. I warrant you; would all women thought no more hurt then thou doost, now sweet villaine, Kate, Kate.
– Kate. I longd for the merry thought of a phesant.
– Greenshield. She talkes in her sleepe.
– Kate. And the foule-gutted Tripe-wife had got it, & eate halfe of it: and my colour went and came, and my stomach wambled: till I was ready to sound [= to swoon], but a Mid-wife perceiued it, and markt which way my eyes went; and helpt mee to it, but Lord how I pickt it, ’twas the sweetest meate me thought.

(* Ud’s life: God’s life)

There is a similar double entendre in On Formio, from Diuine fancies digested into epigrammes, meditations, and observations (London, 1633), by the English poet Francis Quarles (1592-1644):

Formio bewailes his Sins, with the same heart,
As Frends do Frends, when they’r about to part,
Beleeve it, Formio will not entertaine
A merry Thought, untill they meet againe.