'decompress' originates from Latin-derived elements: the prefix 'de-' (from Latin) and the verb 'compress' (from Late Latin 'compressus' / 'comprimere'), where 'de-' meant 'reverse' or 'remove' and 'comprimere' meant 'to press together'.

'comprimere' (Latin) produced 'compressus' (past participle) and then Old/French/Latin-influenced forms led to English 'compress'; adding the prefix 'de-' in modern English produced 'decompress'.

Initially, the root meant 'to press together'; with the prefix 'de-' the combined form came to mean 'to reverse that pressing' — i.e., 'to remove or reduce compression' — and this extended metaphorically to relaxing and to restoring compressed data.