A professional wrestling term, used by smarks.
Professional wrestling, as sports entertainment, requires a certain level of acting. One of the most important acting jobs required of a wrestler is selling the offensive moves of the opponent. Since the wrestlers don't really want to hurt each other, they must look like they are being hurt. One of the all-time masters of selling is Ric Flair.
No-selling, on the other hand, is NOT pretending to be hurt when the opponent uses his offense on you. This is usually considered bad. For instane, your 250lb opponent has just considerately given you a piledriver without actually compressing your spine. Instead of properly selling this move, you get up immediately, without appearing hurt. This is a no-sell, which messes up the delicate suspension of disbelief that is important in enjoying professional wrestling. Unless you are a big WWF Superstar, this no-sell will probably result in the 250lb opponent giving you a piledriver that DOES compress your spine.
A master of the no-sell is the Undertaker.