Second slip

Second slip was, according to the Fifth Doctor, a fielding position in cricket. During the 1981 Ashes series, he suggested that Peri and Erimem go to look for Tegan near second slip. For her part, the American mocked the Doctor's favourite pastime by suggesting, facetiously, "A second slip seems kind of careless after the first one." (PROSE: Graham Dilley Saves the World)

Behind the scenes
Although the Doctor explicitly calls this position a fielding position, it's more specifically a catching position. Slips are behind the batsman, like catchers in baseball, who are hoping to catch the ball after it's nicked the bat, and thereby catch the batsman out.