Edit: Only after having posted this answer did I notice DaG’s comment, which points to exactly the same information; I’d like both to apologize for this, and to make it clear that I had drawn this information from a totally different source (a printed edition of the Commedia). I’ll probably remove this answer after a short grace period (some 48 hours).
The use of the word “bizzarro” dates back at least to Dante Alighieri, who used it (once) in his Commedia, more precisely in Inf. VIII, 62:
Tutti gridavano: «A Filippo Argenti!»;
e ‘l fiorentino spirito bizzarro
in sé medesmo si volvea co’ denti.
We are, here, in the fifth circle, where the sinners of wrath are punished. As a matter of fact, even the Italian annotators of the fourteenth century were rather puzzled by the word, and did not completely agree on its meaning. Boccaccio wrote about it:
Credo questo vocabolo sia solo dei fiorentini,
e suona sempre in mala parte; perciocché noi tegnamo bizzarri
coloro che subitamente e per ogni piccola cagione corrono in ira,
né mai da quella per alcuna dimostrazione rimuovere si possono.
That is, more or less (please forgive my poor translation): “I believe that this word belongs to Florentines alone, and it always has a negative connotation; because we deem bizzarri those who suddenly and for every slightest reason rush into wrath, neither they can be turned away from it, whatever evidence they are given”.