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"). Another theory traces it to an alteration of Medieval Latin floccus, from Latin floccus "tuft of wool," a word of unknown origin. Meaning "outer garment for women or children" is from 1530s. Frock-coat attested by 1819.