Every Monday we will post an entry that hasn’t yet been published with a view towards harnessing the collective onomastic power of the internet. If you have any thoughts about the name’s origin, other variants it might be related to, other examples of its use, etc., please share them in the comments! If you wish to browse other Mystery Monday names, there is an index.
Today’s name is a lovely 14th C feminine name from Picardy. Our single record of it is a Latin genitive form, and involves two of the rare letters of the alphabet — y and z!
We have hypothesized Idosia as a normalized nominative form — we haven’t actually found any instance of this spelling. We would love to have other instances/variants of this name. Do you know of any? Please share in the comments! We also don’t have even the first guess as to what it’s origins might be; if you have any thoughts, we’d love to hear them!
Dictionary of Medieval Names from European Sources 
I propose a Germanic derivation from *Idosintha, the first elment is ID as in Ida (maybe meaning “work”), the second element SINTH is related to Old High German sind “walk, way, passage” (Förstemann 1900 gives modern German translations “Weg, Gang”). The loss of the Nasal and subsequent loss of the dental d/th can be justified for West Franconian forms of the name.
Could it be a variation on Eudosia/Eudoxia?
There is an ‘Ydore’ a little later in the same document (p. 248) that could be the same person/name; this text has a large number of repeated names, with a wide variation in spelling. Also note that ‘Ydozie’ has practically the only ‘z’ in the text, so it could easily be a misreading. (‘Y’ is much more common, especially in Ysabel, Ysabella, Ysabelle, Ysabeau, Ysabiaus.)
Unfortunately there is no index of names.
I mean practically the only ‘z’ at all, not only in names: the only other instance I can find is ‘temoingz’ on p. 189, with a variant ‘tesmoins’.