"coat of arms," late 13c., from Old French blason (12c.) "a shield, blazon," also "collar bone;" common Romanic (compare Spanish blason, Italian blasone, Portuguese brasao, Provençal blezo, the first two said to be French loan-words); of uncertain origin. OED doubts, on grounds of sense, the connection proposed by 19c. French etymologists to Germanic words related to English blaze (n.1).
1560s, "to depict or paint (armorial bearings)," from blazon (n.) or else from French blasonner. Earlier as "to set forth decriptively" (1510s); especially "to vaunt or boast" (1530s), in this use probably from or influenced by blaze (v.2).
