Nahsk was a male alien thief in the 24th century. He had a brother, Krit. Some perceived Nahsk to be mentally slower than his brother, but often it was only an act, as he often played the nicer of the two brothers.
In 2365, Nahsk and associates Hain, Larell, Krit, and Morn, planned and executed the Lissepian Mother's Day Heist, a notorious robbery that occurred on the planet Lissepia, in which one thousand bricks of gold-pressed latinum was stolen from the planet's Central Bank. Though all thieves managed to escape the authorities on Lissepia, Morn double-crossed Nahsk and the others by keeping the latinum for himself. He let him go for fear of being pursued by authorities.
In 2374, the statute of limitations expired for the crime, so he decided to go after Morn with Krit, however, they learned he had died. Discovering he left everything to Quark, they traveled to Deep Space 9. They broke into Quark's quarters and pretended to be his business partners to whom Morn had a debt. After being haggled down to half of the total latinum, Nahsk received an unwelcome painting from Quark, which he promptly used to hit Quark over the head. They stayed close to Quark as he tried to find the money. Eventually, they found out that the other thieves in their group had also approached Quark to get the latinum, and so they all agreed to split it. However, when they all received the latinum in a cargo bay, they pulled phasers on each other. Nahsk heard another comment from Krit about being slow, and that triggered him to turn on his brother, and started firing. He was wounded and arrested with the others by security. No one realized Morn had extracted the latinum and stared it in his second-stomach. (DS9: "Who Mourns for Morn?")
Nahsk and Krit are the only two of their species seen on-screen. Their species is not named in the episode.
The script for "Who Mourns for Morn?" states the pronunciation of Nahsk's name as "NAUSK". It goes on to describe both him and his brother as "well-dressed, but there's something sinister about them that says organized crime." Of Nahsk himself, it says, "Nahsk is the "muscle" of this duo, and a bit slower than his brother."