Groot is the most recognizable member of the Guardians of the Galaxy. Probably because he’s constantly introducing himself. “I am Groot,” he says with varying degrees of inflection to communicate. Groot’s catchphrase is uttered for first time in Infinity Gauntlet #3 by writers Dustin Weaver and Gerry Duggan.
Thanos infiltrates a family of Nova Corps members in order to obtain the reality warping Infinity Gems, one of which is stolen by Gamora and Peter Quill AKA Star-Lord. But these are not the Guardians of the Galaxy. They are cut-throat thieves, who try to cut a gem out of Groot when they initially meet the strange lifeform. The Novas argue in defense of Groot, who strings their words together.
“Eye hmmm grew root,” he declares.
His pronunciation is corrected by Fayne, the youngest daughter of the Novas. Groot’s catchphrase quickly wears thin on Thanos, who decides to kill him first when he makes his move to steal the gems.
This story takes place on Battleworld, a twisted recreation of the universe by a god-like Doctor Doom. Thus it’s uncertain whether it will be canon when the ongoing Marvel reboot resets the world.
Groot’s origin was previously revealed in Guardians of the Galaxy #14. His speech pattern is not an aberration. The creature’s entire species speaks in the same manner. But even that was a retcon (retroactive continuity). Groot spoke intelligibly in his first appearance in Tales to Astonish #13. Unlike the pleasant plant we’ve come to love, Groot’s roots were evil. He came to Earth to abduct humans for experimentation. Fortunately, he was stopped by a brilliant scientist named Leslie Evans, who sicced termites on the tree!