How to make "co-father-in-law" into a preposition in 2 easy steps (and a few hard ones)—a conlanging case study
The Latin word for "father-in-law"—father of one's spouse—is socer /ˈso.ker/. From this the term cōnsocer /ˈkon.so.ker/ is coined for "co-father-in-law"—that is, one child's father-in-law, or one of the two fathers of a married couple.
This word survives realtively unmolested in semantics into Italian consuocero, Portuguese consogro, Romanian cuscru and even Albanian krushk (for which we can only claim 'metathesis'), among others.
Further north, however, the word is eventually lost (though in French suire survives until the second millennium) everywhere except on Borland, where it broadens from the domain of marriage to business partnerships in general.
Already in third-century texts we see cōsocer, cōsocrum used for "the other party to a contract (for example, the other father of a couple who are to be married)" or in the plural cōsocrī, cōsocrōs for "both parties to a contract".
It is likely that by the late fifth century the word has consistent penultimate stress, following the pattern of words like integrum (classically pronunced /ˈin.te.ɡrʊm/ and later on pronounced /inˈteg.rʊm/ [ɪnˈtɛg.rʊ̃]).
At this point or soon thereafter the marital sense is entirely superseded by alternative patraster, patrastrum (giving modern paðrastr [pɐˈʀast(ɐ)]). We can reconstruct ca. 500N cōsocrum as something like [koˈzɔg.ɾʊ].
Writing outside of the Latin used in monasteries is thin on the ground for the next few centuries. We know that by the tenth century (from Hostilian's Histories) the word—now usually spelt cosoȝre and presumably pronounced /koˈzɔɣ.ɾə/—has again broadened, and can be used to refer to a member of any natural pair:
... s il vont senestre eð Marc calaȝre retenoi il cosoȝre...
3s.gen def glove left and Mark knight hold.back-pst def other.in.a.pair
...his left glove, and knight Mark kept the other...
Until this point, cosoȝre has been used exclusively as a noun. A new adjectival use (difficult to distinguish from simple apposition) starts to appear in the record, in examples such as plum cosoȝre [plym koˈzɔʝ.ɾə] "quill (in an inkpot)" and rockel cosoȝr [rɔˈkɛl koˈzɔʝ.ɾə] "distaff (with a spindle)".
So now il X cosoȝre is taken to mean "the X that comes along with other thing in its pair" when used with a single thing, and also "the two Xs that together are a pair".
Moving into the second millennium and the Middle Boral period, two related constructions arise, using cosoȝr [koˈzɔ.jr̩] as an adverb:
Jan eð Mary eurn a cliȝs cosoȝr
Jan ew a cliȝs cosoȝr a Mary
John and Mary went to church as a pair
John went to church in a pair with Mary
This differs from ordinary a couȝl "together" (modern accougl) in implying the two people or things 'go together' in some sense. Very quickly it is established that its use with people implies they are romantically attached (in a distant echo of its etymological roots!).
The first adverbial usage survives to Modern Boral: one might say, for example,
J'acatau y dois eð y scaumel cosogr.
[ˌʝakˈto i ˈdɔjz e‿ði xʊˈmɛw kʊˈzɔ.jɐ]
1s-buy-pst def tablet and def pen as.a.unit
I bought the tablet and stylus as a set.
The other construction falls into line with other preposition-adverb pairs by the seventeenth century, with cosogr a X increasingly attested simply as cosogr X "together with X, in a set with X, in a relationship with X".
And so in my translation of Tim Minchin's You Grew on Me we see
Cosogr tey, jo smarc plu l'aubon.
/koˈzɔ.jʀ̩ ti | ʒo ˈsmaʀk pli loˈbɔn/
[kʊˈzɔjɐ ti | ʝʊˈzmɑːk pli lʊˈbɔn/
together.with 2s.dsj | 1s surface no.more def=morning
(Now that I am) in a relationship with you, I no longer get up in the mornings.
I hope you enjoyed this deep dive! I might do a few more for the more interesting things I had to find out for the Minchin translation in future :)
