Abarim Publications' online Biblical Greek Dictionary
The difficult adverb οπισω (opiso) means backwards, and is usually followed by a genitive form, and thus refers to the back-end of something. In reference to spatial specifications, it often refers to the back of a thing (say, a papyrus roll), or to the hindermost part of some structure or area, implying that there's also a front or forward or prominent part of these things, which our adverb points away from. When our adverb is combined with a movement it often means back or back again, emphasizing a reversal of the direction of the original action.
In reference to time, our adverb (somewhat counterintuitively) refers to the future (i.e. the time that comes after now). Since the past is known and thus continuously regarded, we humans face the past and the past is thus in front of us. That means that the future is behind us, and we back into it, and a movement in the direction of our adverb results in moving backward relative to our field of vision but forward in time. When Jesus famously told satan first (Matthew 4:10) and Peter later (16:23) to get behind him, he both told them to get out of his present field of concern (get out of my sight), and that he would deal with him later (get out of my past and into my future). Conversely, when Jesus told his disciples-to-be to come behind him, he invited them into the future he opened to them (Matthew 4:19).
Since the sun rises in the east, and the east is synonymous with the past (in Hebrew: קדם, qedem, and in Greek: ανατολη, anatole), the sun moves overhead toward the future. Since the sun is the source of light and life on earth, it symbolizes the Word of God from which all knowledge and enlightenment comes (Colossians 2:3). This in turn means that the common working principle of man's cognitive mind is to be wholly aware of the rising of the sun — when cognition begins and things begin to be learned — until man is mature and stops learning new things and merely applies what he knows, not considering the fact that what he knows is part of a day that will ultimately fade as the sun sets behind him. A person who lacks wisdom will fall asleep as his day fades, but a person of wisdom will see the stars for what they are. In psychology we call this Theory of Mind.
Our adverb οπισω (opiso) derives from the noun οπις (opis), which means regard in the sense of an observation, but most often in connection to the deity. This noun was often ascribed to the gods (in the sense that the gods were regarding the humans), in which case it commonly heralded vengeance and calamities. But it was also often ascribed to men (in the sense that men were regarding the gods), in which case it followed veneration and awe. All this suggests that these words were probably commonly associated with the nouns ωψ (ops), meaning eye, and οψις (opsis), a thing seen, regardless of whether they are actually etymologically related.
In a very similar way, the nouns θεατρον (theatron), meaning theatre, a place to see things, and θεωρια (theoria), a sight or "theory", stem from the verb θαομαι (theaomai), meaning to wonder, and are commonly supposed to relate to the familiar nouns θεος (theos), meaning God and θεα (thea), meaning goddess.
Our adverb οπισω (opiso) occurs 37 times in the New Testament; see full concordance