'fast-grown' originates from English, specifically the combination of 'fast' and the past participle 'grown', where 'fast' here functions as an adverb meaning 'rapidly' and 'grown' is from the past participle of 'grow'.

'fast-grown' was formed in modern English by compounding 'fast' + 'grown' (the past participle of 'grow'). The components 'fast' and 'grow' are Old English in origin, but the particular compound usage is attested from modern agricultural and industrial usage (19th–20th century onward).

Initially, 'fast' in Old English primarily meant 'firm, fixed' ('fæst'), but over time it developed adverbial senses including 'quickly' or 'rapidly'; in this compound the sense 'quickly' dominates, yielding the meaning 'grown quickly'.