Gif Central boasts hundreds of funny animated gifs. Only the best of the best make it onto this site, with most of them having an accompanying source. No tumblr-esque one-second-long disappointments. No obscure titles or descriptions.
This is animated gif of a man beating a dead horse. This can be used as a reaction whenever somebody tells a joke that has been told a million times before (hence the phrase). In this gif, the man is literally beating the horse with his fists.