First slip was a fielding position in cricket. Peri implied their existence at the 1981 Ashes series by telling the Fifth Doctor, "A second slip seems kind of careless after the first one." (PROSE: Graham Dilley Saves the World)
Behind the scenes Edit
Although the Doctor explicitly calls second slip — and therefore by implication first slip — 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.