Ooh, thanks, Bingley:
Armscye (Armseye)

It's not a term that is particularly medieval, but it gets tossed around a lot in medieval clothing discussions. It means 'Armhole', or that roundish place in the body of a garment that the sleeve gets set into.

OED "Scye - The opening in a coat into which a sleeve is inserted. 1st listed use is 1825 JAMIESON Suppl. s.v. Sey, The sey of a gown or shift is the opening through which the arm passes. Etymology is listed as "A use of a Scots and Ulster dialect word (written also sey, sci, si, sie, sy in glossaries) meaning ‘the opening of a gown, etc., into which the sleeve is inserted; the part of the dress between the armpit and the chest’ (E.D.D.); of obscure etymology.
Armseye is listed in a description of 'Dolman' (sleeves) in the OED, dated to 1934.

From:
http://www.personal.utulsa.edu/~marc-carlson/cloth/glossary.html