Meaning

noun the ornamental front of a woman's bodice or shirt

noun the front of man's dress shirt

noun a metal breastplate that was worn under a coat of mail

originBorrowed from French plastron, from Italian piastrone, augmentive of piastra (“breastplate”), from Latin emplastrum (“plaster”), from Ancient Greek ἔμπλαστρον (émplastron), from ἔμπλαστος (émplastos, “daubed, plastered”), from ἔμπλασσειν (émplassein, “to mould, form”).

Etymology from Wiktionary, CC BY-SA 3.0