In mathematics, a line or curve that acts as the limit of another line or curve.
For example, a descending curve that approaches but does not reach the horizontal axis is said to be asymptotic to that axis, which is the asymptote of the curve.
You've seen those curves graphed. Often they have a gradual slope until they get close to their limit, their asymptote, and then the slope of the curve gets dramatic. I've always seen it as a metaphor for so much in life. The closer you get to perfection, the more effort it takes to make smaller and smaller increments of progress. The better things get, the harder and harder it is for them to get better. That's why its funny.
*Also a reference to the pilot episode of Arrested Development.