The Russo Brothers held a Q&A at the Smithsonian Air & Space Museum and answered a flurry of fan questions. One of which was a keen fan wanting to know what Groot says with his one “I am Groot” line in Avengers: Endgame. We only hear Groot speak once in the film and it comes right at the end. Unless you can speak Groot (the language) like Rocket and Thor can, it always sounds as though Groot is saying the same 3 words, but he is actually saying different things every time. Vin Diesel, the voice of Groot, even gets his script with what Groot is saying written in English so the actor has context when delivering the lines. with So what does the beloved character say with his one line in Endgame?
Whilst Peter Quill and Thor squabble about who the real Captain of the Benetar spaceship is, Drax suggests that the 2 fight to see who should take the helm. Rocket then offers that he has blasters or they could use knives and Groot says… “I am Groot” which translates to “Yes, I like knives.” So much like the rest of the Guardians on board the ship, Groot also wanted to see the Quill and Thor fight.
Thanks to reddit user zachzhou630 for providing the information on the Marvel Studios subreddit.