'slow-matured' originates from English, specifically formed by combining the adjective 'slow' (from Old English 'slāw') and the past-participle form 'matured' (ultimately from Latin 'maturare'), where the root 'matur-' meant 'ripe'.

'slow' changed from Old English 'slāw' into modern English 'slow'; 'matured' evolved from Latin 'maturare' through Old French/Medieval Latin and Middle English forms of 'mature' before becoming the past participle 'matured' in modern English. The compound meaning arose by combining these elements in modern usage.

Initially the components meant 'slow' + 'ripe/mature'; over time their combination came to be used specifically to describe things that reach maturity at a slower pace, i.e., 'taking longer to mature.'