leg-of-mut·ton(lĕg′ə-mŭt′n, lĕg′əv-) or leg-o'-mut·ton (lĕg′ə-)
Resembling a leg of mutton in shape; tapering sharply from one large end to a point or smaller end, as a sleeve or sail.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
(Clothing & Fashion) (modifier) (of a sail, sleeve, etc) tapering sharply or having a triangular profile
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
1. having the triangular shape of a leg of mutton: a leg-of-mutton sail.
2. (of the sleeve of a dress or blouse) full and puffed from shoulder to elbow and then tightly fitted from elbow to wrist.
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.