Definition of "merrythought"

One puts ye merrithought on his nose (slightly) like a paire of spectacles, and shakes his head till he shakes it off his nose.

1687, John Aubrey, Remaines of Gentilisme and Judaisme, page 93

All Birds want the Channel-bones in the Breast, instead whereof they have a bone, which we call the Merry thought.

1688, Randle Holme, The Academy of Armory‎, Chester, Book 2, Chapter 13, p. 307

Mrs. Knipp. (picking up from Ming's plate the merrythought on which her eye has just fallen, and holding it up). A merrythought. Who will pull this merrythought with me ? Nay (as they all lean forward), 'tis an affair of two not four. (They draw back simultaneously, eying each other sheepishly when Mrs. KNIPP laughs at their confusion).

1906, William Price Drury, Richard Pryce, A Privy Council: A Comedy in One Act, page 22