The adjective εχθρος (echthros) means hostile, or something so strikingly unfamiliar that one naturally reacts with repulsion. It stems from the noun εχθος (echthos), hate, which in turn is identical to the adverb εχθος (echthos), outside, and which is really an alternate spelling of εκτος (ektos), or ex-tos, "out from it". Another word of note is the adverb εχθες (echtes), meaning yesterday, which is an expanded version of χθες (chtes), also meaning yesterday (see below).

Contrary to common perception, darkness is not the opposite of light but the absence of it. Likewise, ignorance is not the opposite of wisdom but the absence of it. And likewise, hate is not the opposite of love but the absence of it. As we discuss at some length in our article on the noun αγαπη (agape), meaning love, "love" is not a mushy feeling but a measurable force that brings about synchronicity between people's individual models of reality. It drives the formation of symbols, and thus words, language and science. And since God's invisible attributes, eternal power and divine nature are clearly seen through the formal measurements of the observable universe (Romans 1:20), it makes humanity divine (also see our article on οινος, oinos, meaning wine). Our adjective εχθρος (echthros) describes the absence of all that (and see Isaiah 45:7 to find out where darkness in all its forms comes from).

Our adjective εχθρος (echthros) is a social word and describes outcast (see our article on the adjective πτωχος, ptochos, destitute, or more elaborate: having no supporting family or friends and being too ill or weak to work). It describes the failure to recognize kinship and thus a broadening of what's known as the "uncanny valley", where someone's unfamiliar appearance generates feelings of revolt and apprehension (Isaiah 53:3). Everybody can be nice to friends and family but the whole trick to being an enlightened human being is the ability to rise above one's reflexes and engage a stranger in a way that is both safe and formative (Matthew 5:43-48).

Our adjective occurs 32 times in the New Testament, see full concordance, and from it derives:

The adverb of time χθες (chthes) means yesterday (John 4:52, Acts 7:28 and Hebrews 13:8 only). In the classics, it occurs also in the form εχθες (echthes), but its similarity with the above appears to be accidental (but, to anybody creative enough, perhaps not without portent: see Matthew 6:34).

Our adverb is thought to derive from the same ancient and widely attested Proto-Indo-European root that gave English the "yester-" part of "yesterday". What that root may have originally expressed can no longer be established, but perhaps it had to do with a pronoun (like "that"), which complemented the "this" of "to-day". Still, our Greek word χθες (chthes) does not have the word for "day" in it, and may technically refer to the most recent past in general, with less strict adherence to specific calendar dates. This same PIE root gave Latin its word heri, which likewise means yesterday or a time just past.