1
general::
Ceremonial dress worn at graduation exercises; by extension, the academic community ( Also see TOWN AND GOWN). For example, Mary was very proud when she received her cap and gown for commencement. [Mid-1800s]
American Heritage Idioms
2
general::
Phrase(s): cap and gown
the academic cap or mortarboard and the robe worn in formal academic ceremonies. • We all had to rent cap and gown for graduation. • I appeared wearing my cap and gown, but I had shorts on underneath because it gets so hot at that time of year.
McGrawhill's American Idioms And Phrasal Verbs