'long-grown' originates from Modern English, specifically as a compound of the adjective 'long' (from Old English 'lang', where 'lang' meant 'long') and the past participle 'grown' (from the verb 'grow', from Old English 'grōwan').

'long-grown' developed from earlier phrasing such as 'long grown' (two separate words) and later became hyphenated in modern English usage; the component 'grow' evolved from Old English 'grōwan' through Middle English 'growen' to Modern English 'grow', with the past participle 'grown'.

Initially it was a literal compound meaning 'having grown for a long time'; over time it has retained that literal sense but has also been used more generally to mean 'long-established' or 'having existed/developed over an extended period.'