I was poking around some research into Merovingian/Frankish and Medieval French names [as you do] and started getting excited to find names I had found appearing in English records that looked as though they had come over with the Conqueror and were subsequently thoroughly Anglicised - Otwell, for example, had always bugged me until I discovered Otuel.
Then I began a theory about the female name Toussaine with its male version Toussaint/Tosseyn, which got me quite excited.
I don't have any books on French names so I tried the baby name origins online which are usually a good starting point if you cross reference; and discovered that Toussaine had no meaning and Toussaint was given as 'All Saints' for being born on 'All Saints' Day' and I'm thinking, come on, it's not going to be that easy - All Saint's Day wasn't called that back in Middle Ages, it was Hallowmas.
So here goes the theory:
Frankish naming policy used a bipartite naming system with a prefix and suffix of 'qualities'; the same suffixes were used for both sexes and women had either to rely on the limited number of female suffixes or have -a appended to a male name.
Theud- prefix meaning folk; often becomes Teu'Te/Ty etc[cf Theodoric's transformation to Tedric and Theudebald to Theobald to Tybalt]
-sind a suffix meaning a road or pathway. We know how 'd's at the end of words get abused and, in French where final letters are not pronounced, are easy to get lost entirely. Equally 'd' becomes 't' or either of these letters is cheerfully added.
So, speculatively, a Frankish/Merovingian name Theudsind or for female Theudsinda.
What I found in the eleventh century was a female name Teuscenda. I'm not that bothered about the excrescent 'c' since this appears often enough in other names like Melisende to Meliscende, Acelin to Ascelin and so on.
In 1292 I have an appearance of Tycelin and Tyce which may be diminutives - Tyce a shortening, and the -lin suffix as a well-established diminutive, the Ty- prefix, like Tybalt, apparent as a use as Theophania [of different origin, being Greek, with a coincidental sound] becomes Tyfainne, Typhanete, Typhenon Theffanie etc
The data base of female names is unfortunately sketchy in the middle ages, Toussaint and Tosseyn appear in the fifteenth century in male names [also in mid century possibly as Tassin but that may be dodgy] so the next appearance of the name I have is in its modern form Toussaine in the sixteenth century.
What do you think - Theudsinda -Teuscenda - Toussaine?
and equally Theudsind - Tosseyn - Toussaint?
A rummage through random aspects of the past that interest me and may be of use or interest to other readers and writers of period fiction. Please note that the stories featured and my artwork for the covers are copyright; and have the courtesy to ask permission if you wish to use anything that is mine, and duly acknowledge it if you do.