Excretus of Borg was the identity assumed by Ensign Brad Boimler after he was assimilated by the Borg within Borg Encounter, one of the Starfleet holographic training drills given aboard the USS Cerritos in 2381. Having already spent hours in the simulation trying to perfect his score, Boimler had been ordered by Captain Carol Freeman to prolong the drill by any means necessary, and was thus forced to stay aboard a Borg cube until his capture by Borg drones.
As Excretus, Boimler was insensate and unable to exit the simulation of his own will: he had to be manually removed from the simulator holopod by Commander Andy Billups and Lieutenant Shaxs, whereupon the program ended and his body returned to normal. However, the mental effects of the experience lingered, such as an imperative to assimilate. Boimler would later lament that the Borg "took everything that [he] was". (LD: "I, Excretus")
The name "Excretus of Borg", like "Locutus of Borg", is derived from Latin. "Locutus" means "having spoken" or "he who has spoken." "Excretus," on the same model, means "having separated" or "he who has separated"; however, "Excretus" also carries overtones of the English word "excrete", meaning to defecate.