The loser gesture is used as a sign of mockery towards those who have lost a challenge or a game. It became popular in the 1990s thanks to the film Ace Ventura. Meaning: if someone addresses you with this gesture, they want to communicate something like: "You lost big time!". Or, it could be a quote from the song Loser by Beck. In which case, you immediately join a group of people who are a bit of a loser, but cool, deliberately countercultural and vaguely hipster.