This item was on the 10-2011 LoAR
1: Armegardj Sigurdsdottir - New Name
Submitter desires a feminine name.
Culture most important.
Armegardj - A version of Ermengard dated to 1383 from http://www.ellipsis.cx/~liana/names/swedish/smp/ermengard.html
Sigurdsdottir - Found in "Vanhat nimityyppimme (Finnish Names)" by Rouva Gertrud at http://heraldry.sca.org/laurel/names/FinnishNamesArticle.htm
Also, St. Gabriel Report #1799 (http://www.s-gabriel.org/1799) states: "The name <Sigurðr> was extremely popular in Norway and Iceland right through the Middle Ages in a variety of forms; the modern <Sigurd> appeared by about 1300. The <ð> stands for the Norse letter "edh".  The genitive form was originally <Sigurðar>, making the patronymic <Sigurðar son>. However, we have found <Sigurds> in 1346, so the patronymic byname <Sigurdsson> would be appropriate after then." Source: Lind, E.H., _Norsk-Isla:ndska Dopnamn ock Fingerade Namn fra*n Medeltiden_ (Uppsala & Leipzig: 1905-1915, sup. Oslo, Uppsala and Kobenhavn: 1931)
The patronymic surname <Sigurdsdottir> is constructed in accordance with practices outlined in Geirr-Bassi "Old Norse Name" regarding construction of patronymics.
The statement for the byname based on the Finnish name article is not quite accurate as the form Sigurdsdottir does not appear in that article, only the masculine given name Sigurd in the name of Sigurd Whenapha dated to 1379. The examples in that article never use the western Scandinavian form -dottir but note that patronymics can be formed either entirely from Finnish forms or at times using Swedish names and patronymic elements with the example of Maijsa Lucija dotter given which clearly shows the Swedish form -dotter. As documented, the name appears by precedent to be a step from period practice because of the admixture of the Swedish given name with the byname which uses forms that appear to be primarily Old Norse given the patronymic suffix used in the byname. (Per Friedemann's article "Swedish Feminine Given Names from SMP" cited for the given name, a fully Swedish patronymic form would use either dotter or dotther.) I strongly suspect that, given the statements in the Finnish name argument, you could remove the step from period practice by changing the byname to Sigurds dotter (one letter and not much sound change) either separately as in the examples or coalesced by dropping the space.