dementia

Latin dēmentia was built from dē- ("away, from") plus mēns ("mind"), with the noun-forming suffix -ia attached to the stem dēment-. English borrowed the word directly from this Latin form.

The facts available don't trace a shift in meaning beyond the Latin coinage and its borrowing into English.

Dementia shares its root mēns, "mind," with any English word built on that same Latin stem — the mind is baked right into the word's core.