Frock (n.)
mid-14c., from Old French froc "a monk's habit; clothing, dress" (12c.), which is of unknown origin; perhaps from Frankish hrok or some other Germanic source (compare Old High German hroc "mantle, coat;" Old Norse rokkr, Old English rocc, Old Frisian rokk, German Rock "a coat, over-garment"). […]

Source : Etymology online

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