What is the root of the word apathy?

Etymologically, the word comes from the ancient Greek language. In Greek it is 'apatheia' that comes from 'apathēs', i. e. 'without feeling'. It consist of the prefix 'a-' (meaning 'without') and 'pathos' ('feeling' or 'passion').

Historically, the word appeared (or at least popularised) in the context of Stoic philosophy, in which apathy was considered the preferred state of mind guaranteeing the most perfect and virtuous life possible.