Which season 6 episode reveals why Wylis became known as Hodor?

The season 6 episode that reveals why Wylis became known as Hodor is “The Door.”

“The Door” is the fifth episode of season 6 and the 55th episode overall. Its closing sequence uses Bran Stark’s visions to connect the past and present: young Wylis witnesses his future self holding a door shut while Meera Reed repeatedly cries, “Hold the door!”

Because Bran is simultaneously connected to Wylis in the past and Hodor in the present, the command overwhelms the young stable boy. “Hold the door” gradually collapses into “Hodor,” leaving Wylis with the single word he will use for the rest of his life. The scene also shows Hodor’s death as he protects Bran and Meera from the wights.

A common mix-up is “Home,” which contains an important earlier flashback showing that Hodor was once a normally speaking boy. But “Home” does not explain the origin of his name; that revelation belongs specifically to “The Door,” written by David Benioff and D. B. Weiss.