Geefy, whose full name would presumably be Geefy Goofy, is, or possibly was, an anthropomorphic dog.
Geefy is the younger brother of Goofy. Thus, it can be assumed that he is a son of Amos and Maw Goofy. According to his older brother Goofy, he "had the craziest laugh you ever want to hear". This may have been a little hypocritical of the elder Goofy brother, as Geefy's laugh very much resembled Goofy's laugh, though the younger Goofy brother's laugh was much higher-pitched.
While hosting a television special in 1964, Goofy referred to his younger brother Geefy in the past tense. This might imply that Geefy had died by 1964, though it's not known that this is true. On the other hand, Goofy may have used the past tense to make it clear that Geefy no longer had "the craziest laugh you ever want to hear". That Geefy's laugh could have changed without him dying is not impossible.
Behind the scenes
Geefy was first mentioned in the 1964 album Goofy's TV Spectacular. However, he may have actually first appeared in a 1953 comic strip written by Bill Walsh and drawn by Manuel Gonzales known on INDUCKS as ZM 53-06-07. A photograph of Goofy's family can be seen in this strip, and, although it is unconfirmed, it's possible that one of the relatives seen in the photograph is in fact Geefy.