To the Mal-

content.

IF by the Cynic tooth you are pierced, original: "Cinico dente." A reference to the Cynic philosophers (from the Greek "kynikos," dog-like) known for their biting, "barking" social critiques.
Complain of yourself, you barbaric dog: original: "perro." Bruno uses the Spanish word for dog to add a foreign, perhaps harsher, flavor to the insult.
For in vain you show me your staff and iron, original: "baston, et ferro." Likely referring to the weapons of a critic or the tools of a shepherd/guard dog.
If you do not refrain from showing me spite.
Because with wrong you came at me directly,
Therefore I tear your skin and lay you open:
And if it then happens that I strike down my body,
Your disgrace is written in diamond. Suggesting that while the author may suffer physically or be "downed," the critic's shame will be permanently recorded in something as indestructible as diamond.
Do not go naked to take honey from the bees.
Do not bite if you know not if it be stone or bread.
Do not go barefoot to sow thorns.
Do not, as a fly, despise the spider’s webs.
If you are strong, do not follow the frogs,
Flee the foxes, O chicken-blooded. original: "sangue di galline." Literally "chicken-blooded," a common idiom for a coward.
And believe the Gospel,
Which says with good zeal,
From our field he reaps repentance:
Who sowed there the seeds of error. This "coda" or tail of the sonnet warns that those who spread misinformation will eventually have to harvest their own regret.

Cynic (from "Cinico"): A member of a school of ancient Greek philosophy that practiced a rigorous, often abrasive, rejection of social conventions; often associated with dogs.
dog (from "perro"): A Spanish term for dog, used here as a derogatory address to a critic.
Gospel (from "Euangelo"): Referring here to a general moral truth or "good news" regarding justice and consequence.
repentance (from "penitenza"): The act of regret or making amends for one's errors.
tailed sonnet (from "sonetto con la coda"): A variation of the sonnet form that adds extra lines (a "tail") at the end, often used for satirical or humorous purposes in Italian literature.