Matador was the Spanish word for "killer"; it was used for naming the bullfighter ("torero") who, in the outcome of some bullfighting, actually killed the bull.
A painting of a matador was discovered aboard an old derelict freighter in 2373, and was one of the items auctioned off at Quark's. According to Quark, this painting, identified as Lot 49, was "a rare example of a 20th century Human art form, acrylic on black velvet. This image was the inspiration for the flag of the Martian colonies in the 22nd century." Morn was the highest bidder and purchased the painting. (DS9: "In the Cards")
In 2374, the painting was one of the few items to be found in Morn's spartan quarters. An unhappy Nahsk later smashed the painting over Quark's head, thereby destroying it. When Quark lifted the painting off his shoulders, he found a hidden isolinear rod in the fabric, hidden there by Morn. (DS9: "Who Mourns for Morn?")