What does this word mean? The first person to correctly answer the question will get glory, and undying respect and admiration. No fair peeking.

Here's the answer to yesterday's puzzle -- cosinage is not related to cosines, cosigning, cozening, cosseting, or the Slavic "syn" (meaning "son"), or the Latin "sinus." Rather (isn't it obvious?) it's what happens when you can't get an ayle, besayle, de avo, de proavo, or mort d'ancestor. ("Very barbarous names," says Justice Duncan in Witherow v. Keller, 11 Serg. & Rawle 271 (Pa.).)

OK, it isn't quite that hard, since it is indeed (as some commenters noted) related to cousins and consanguinity. KeithKW was the first to note the consanguinity link, so the promised glory &c. goes to him. I turn to Sir William Blackstone (the quotation is from my 1847 American edition, though the link is to a different edition):

[An action by writ of] assize is a real action which proves the title of the demandant merely by showing his or his ancestor’s possession ...

If the [ouster of the demandant] happened upon the death of the demandant’s father or mother, brother or sister, uncle or aunt, nephew or niece, the remedy is by an assize of mort d’ancestor, or death of one’s ancestor. This writ directs the sheriff to summon a jury or assize, who shall view the land in question, and recognize whether such ancestor were seized thereof on the day of his death, and whether the demandant be the next heir; soon after which the judges come down by the king’s commission to take the recognition of assize: when, if these points are found in the affirmative, the law immediately transfers the possession from the tenant to the demandant. If the abatement happened on the death of one’s grandfather or grandmother, then an assize of mort d’ancestor no longer lies, but a writ of ayle or de avo; if on the death of the great-grandfather or great-grandmother, then a writ of besayle or de proavo; but if it mounts one degree higher, to the tresayle, or grandfather’s grandfather, or if the abatement happened upon the death of any collateral relation other than those before mentioned, the writ is called a writ of cosinage or de consanguineo.

By the way, though the main entry in the Oxford English Dictionary for this term is "cousinage," the dominant spelling in English and American cases has indeed been "cosinage." The origin is indeed "cousin," but, as the OED points out, "In mediæval use, the word [cousin] seems to have been often taken to represent L. consanguineus," though the actual origin of "cousin" is from the Latin consobrinus.]

More from Justice Duncan:

On the general doctrine of disseisin, Lord MANSFIELD has said, it was once well known, but is not now to be found; the more we read, the more we shall be confounded. Taking it for granted, that what would confound Lord MANSFIELD, would overwhelm me, I never have puzzled myself with this abstruse subject, and I never shall; the study of it neither affords profit nor pleasure, and I have no ambition to be wiser than Lord MANSFIELD.