Answer
The moniker "Monophthalmus," meaning "one-eyed," was bestowed upon Antigonus due to a battle injury that resulted in the loss of his right eye during a conflict with Eumenes at the Battle of Ipsus in 301 BCE. This distinguishing physical characteristic became synonymous with his identity, leading to the widespread use of the epithet "Monophthalmus" to refer to Antigonus.