it is a term that seemed to be used similarly to مسمار جحا, as it is all about a reason for commiting and doing somethig that might attract questioning.

فالثوار يسيطرون على المدينة، ووجد أن إقامة الحد تزيد من ثورتهم فلم يُقم الحد، مما أعطى معاوية فرصة إتخاذ ما اسمى قميص عثمان أي الاحتجاج بضرورة إقامة الحد على قتلته

http://www.imtidad.com/home/printview/1482

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Note added at 1 hr (2007-01-19 23:52:39 GMT)
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For a better understanding of its real meaning:

Othman's Shirt. To history-minded Arabs, the shirt-waving guerrillas recalled a major battle in Islamic history. Thirteen centuries ago, a Damascus governor named Mu'awiya, vowing to avenge the murder of the Caliph Othman, carried Othman's bloody shirt as a battle flag. Actually, Mu'awiya hoped to make himself Caliph. Ever since, Arabs have described self-aggrandizement in the guise of vengeance as "waving the shirt of Othman."

http://www.time.com/time/magazine/article/0,9171,878407,00.h...

Depending on in which way the text is going. Used to indicate that the expressed seemingly-benign motive is false while the true hidden motive is evil. Colleagues got it right about the historical reference.