A fast ball which pitches short but (due to the vagaries of the wicket surface) does not bounce high is very difficult to defend against, for the batsman will usually expect the ball to bounce high in this situation; this can often lead to the batsman being out bowled or lbw when they leave a ball which appeared to be rising over the stumps but stayed low. Such deliveries are not called long hops, due to the difficulty they cause the batsman.
The term 'long hop', like the term 'Full toss', has passed into slang as a description of an argument or insult which is easily refuted or responded to.