Source: http://www.motherjones.com/media/2014/06...

According to George R.R. Martin, who wrote the epic books that inspired the HBO show, the 7-foot-tall Hodor could only say one word—"Hodor"—and everyone therefore tended to assume that was his name.

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Hodor's combination of impoverished speech production with relatively normal comprehension is a classic, albeit particularly severe, presentation of expressive aphasia, a neurological condition usually caused by a localized stroke in the front of the brain, on the left side. Some patients, however, have damage to that part of the brain from other causes, such as a tumor, or a blow to the head.

Martin doesn't provide any details regarding whether Hodor suffered a traumatic brain injury as a child. But his symptoms are consistent with this type of disorder, also dubbed Broca's aphasia after a 19th-century French physician named Paul Broca. Broca described a patient who had a lesion in the left frontal part of his brain and who could only say one word: "Tan." And just like Hodor, Broca's patient came to be known as "Tan"—the single word that he could utter voluntarily—even though it wasn't his actual name.