inside, or out from inside: the interior of something, usually set against the outside that is on view
Greek could tack an ending meaning "from" onto a place word, and that is how the plain word for "inside" (ἔσω) became this one. So its oldest force is "out from inside", which is exactly what it does in Mark 7:21, where evil thoughts come out of a person's heart. By the first century that force had softened, and most of the time the word simply marks where something is: a scroll written on the inside (Revelation 5:1), or eyes on the inner side of a creature's wings (Revelation 4:8).
What makes the word memorable is how often it is half of a pair. Six of its twelve appearances stand right beside its opposite, "outside" (ἔξωθεν), and the pairing always sets a surface against what is behind it:
- the outside of a cup scrubbed clean while the inside is full of greed (Matthew 23:25)
- tombs that look beautiful from the street and hold bones inside (Matthew 23:27)
- people who look upright to everyone watching and are full of pretending underneath (Matthew 23:28)
- fighting on the outside and fear on the inside (2 Corinthians 7:5)
By itself the word carries no verdict. It only says "on the inside", and the sentence decides whether that inside holds greed, bones, fear, or the eyes of a worshipping creature. Twice in Luke it stands alone as a thing in its own right, "the inside", so that Jesus can ask whether the God who made the outside of a person did not also make the inside (Luke 11:40).