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Related to Eton jacket: mess jacket
A waist-length, traditionally black jacket that has wide lapels and is cut square at the hips.
[After Eton College, England.]
(Clothing & Fashion) a waist-length jacket with a V-shaped back, open in front, formerly worn by pupils of Eton College
a boy's black waist-length jacket with wide lapels and an open front, orig. worn by students at Eton College.