noun
American armhole
A wasei-eigo term used in fashion and sewing to refer to a set-in sleeve with a deep, rounded armhole, typical of American-style casual wear like T-shirts.
wasei-eigo (Japanese-coined English), from American sleeve
このTシャツはアメリカンスリーブで、動きやすい。
This T-shirt has American armholes, so it's easy to move in.
パターンではアメリカンスリーブの縫い方を説明している。
The pattern explains how to sew an American armhole.