Academic dress or robes worn by students and faculty at universities and colleges, such as gowns and mortarboards.
From academical (adjective) used as a noun, following the pattern where adjectives describing clothing become nouns (like 'formals' for formal wear). This usage is primarily British and historical.
The word 'academicals' perfectly captures how British English treats formal academic dress—it's used as a plural noun in Oxbridge and other universities, so students 'wear their academicals' to formal events, making clothing itself sound scholarly.
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