The story
After Harvey Smith, a British show jumping champion, who made a V sign to the judges in 1971 after a near perfect round. Smith had previously been in an argument with one of the judges, who had said that he (Smith) would not win. Having proved the judge wrong, Smith made the insulting gesture, although in the ensuing controversy he claimed that it had been a V sign of victory. https://www.theguardian.com/sport/2009/aug/15/harvey-smith-shows-v-sign
Etymology from Wiktionary, CC BY-SA 4.0.