The alien being who became known as the Impossible Man was born on the planet Poppup and is a member of the Poppupian race. Living on a world fraught with danger the Poppupians evolved into a state where they could instantly transform into anything their imagination could create. The race also developed into a collective species, with a group mind and that their collective memories and genetic history could be preserved so long as only one of their race endured. By the Impossible Man's accounts the people of Poppup had no named because they "knew who they were".
Soon enough the Poppupians grew bored with living on their own planet and one of their number left the planet Poppup to find an ideal vacation spot to entertain them. This Poppupian eventually landed on Earth for the first time roughly 10 years ago shortly after the birth of the Avengers. The alien then began causing trouble in New York City, due to his prankish behaviour and his inability to fully understand Earth culture, however this alien relished the entire experience. Soon the Avengers were called in to clash with the alien, who gave them chase across town. During the battle, Hawkeye called the alien "impossible" and he became commonly referred to as the Impossible Man ever since.
Comics design of Impossible Man