'slowly-inflated' originates from English, specifically a compound of the adverb 'slowly' (from Old English 'slaw' / 'slǣw') and the verb 'inflate' (from Latin 'inflāre'), where 'slaw' meant 'slow' and 'inflāre' meant 'to blow into'.

'slowly' developed from Old English 'slaw' -> Middle English 'slow', while 'inflate' entered English via Latin 'inflātus'/'inflāre' often through French influence (e.g. Middle French) before becoming the modern English 'inflate'; the compound construction combining an adverb + past participle is a common modern English formation.

Initially, the roots meant 'slow' and 'to blow into' respectively; over time they combined in modern English to form a descriptive compound meaning 'inflated slowly' with little change in the core senses of the parts.