The putting outward of what was inward. Expression is not decoration; it is how an interior becomes shareable.
Latin exprimere, ex- (out) plus premere (to press); the noun for the putting-outward.
The same pressing that leaves an impression or a print: exprimere names a squeezing-out under force, not a decorating-over. Expression is the mechanism by which what pressed for release actually reaches a surface another can read.